Analysis by Z. Byron Wolf, CNN

(CNN) -- Let's take this moment to pause and take stock of the jetsam that has been jettisoned from President Donald Trump's Cabinet and compare it with what hasn't.

Beware the effort to make full sense of the President's hiring and firing decisions or to explain why one person (Rex Tillerson, for example) doesn't last, while another person (Scott Pruitt, say) does. So far, at least. Pruitt could be gone tomorrow, for all we know.

But there are some themes worth examining here. Notably, that what seems to get a Trump Cabinet official forced out is not scandal but disagreeing with the boss.

That's why Secretary of State Rex Tillerson and VA Secretary David Shulkin were both let go via tweet. They both disagreed with Trump on real policy moves. Trump said Tillerson didn't want to end the Obama-era Iran nuclear agreement. Trump does.

So no hard feelings, but there's the door.

Tillerson's very simple policy dispute with Trump (exacerbated, perhaps, by those reports that Tillerson had called Trump a "moron" behind closed doors), seems tame compared to the high-flying scandals ensnaring other Cabinet members.

It's a wonder Shulkin made it as long as he did; the subject of an IG investigation over his travel. But it was a hard policy issue -- how much to privatize the health care of veterans -- that ultimately seems to have created an irreparable rift between Shulkin and the White House. Now they can't even agree on whether Shulkin was fired or not.

There has been no such rift for EPA administrator Pruitt, despite a string of embarrassing stories about his first-class flights, his apparently cozy low-rent housing situation courtesy of the spouse of an energy lobbyist, and more.

If an embarrassing scandal is going to claim a Cabinet secretary, the smart money is on Pruitt. But we'll see.

For all that, Pruitt still has his job. And he got a call of encouragement from the President on Tuesday. Why? Who the heck knows! But with Trump, it's worth mentioning that Pruitt is executing his duties as Trump sees them -- dismantling the US regulatory state -- quite well. He announced Tuesday a plan to revisit car emissions standards put in place during the Obama presidency.

Neither Housing and Urban Development Secretary Ben Carson nor Education Secretary Betsy DeVos appear to have crossed Trump on a policy idea. They both still have their jobs, despite some embarrassments. DeVos gave a less-than-stellar interview to "60 Minutes." Carson came under fire when his office tried to spend taxpayer money on extravagant redecoration of his HUD office. (Trump also might also be loath to lose the only African-American member of his Cabinet and one of the few women).

The President wasn't too torn up about losing Gary Cohn, who was not a Cabinet member as director of Trump's National Economic Council. But Cohn was a principal and top policy mind at the White House. He and Trump differed over tariffs the President unilaterally enacted on steel and aluminum imports, and then Cohn resigned.

Trump never gelled with national security adviser H.R. McMaster, and replaced him with John Bolton, who has much more aggressive views on foreign policy, in line with Trump's own.

There are obviously exceptions. Trump's first HHS Secretary Tom Price resigned under pressure back in September of 2017 during a private plane scandal in which he spent hundreds of thousands of taxpayer dollars on private transportation.

But other Cabinet secretaries, including Pruitt, Shulkin, Treasury Secretary Steven Mnuchin and Interior Secretary Ryan Zinke all have had travel issues.

What helped push Price out the door was his attachment to the failed efforts by Republicans to repeal Obamacare. Trump even said during a speech to Boy Scouts in July that if Price couldn't get the votes to repeal Obamacare, he'd be gone.

"He better get them. Oh, he better," Trump said. "Otherwise I'll say, 'Tom, you're fired.' "

The wild card in all of this is Jeff Sessions, the attorney general who was an early and vocal Trump supporter but who Trump now regrets appointing. Sessions' big problem is that he blinked in the face of public scrutiny and recused himself from any sort of Russia election meddling investigation. That set off the series of dominoes that gave us Robert Mueller's special counsel investigation, which has already gotten guilty pleas from former Trump campaign aides and shows few signs of slowing down.

Former White House Chief of Staff Reince Priebus has said he had to chase down Sessions outside the White House and convince him not to quit at one point. Trump has made sport of publicly humiliating Sessions to the point that Sessions has pushed back at times. But Sessions seems to have weathered the storm for now and is busy carrying out Trump's hardline immigration policies and engaging in political war with California.

Priebus, by the way, took the fall for a string of legislative failures and leaks from the White House. Former chief strategist Steve Bannon was forced out after he contradicted Trump on North Korea and overstated his power. Failure to enact Trump's agenda -- or unwillingness to do so due to policy differences -- has been the downfall time and again for members of the President's team.

So, Sessions and Pruitt are in, for now. And Tillerson and others are out. It's a lesson other Cabinet members might want to learn. They'll need to keep up with the boss if they want to stay put.