Today is the 11th anniversary of Steve Ballmer's tenure as CEO of Microsoft.

He's been controversial to say the least, with tales of temper tantrums, accusations that he's missed big opportunities, and an occasionally crazed public demeanor. Outsiders often wonder how he can still be running the company.

What a lot of outsiders don't understand is that Ballmer is a numbers guy. He aced the math portion of the SAT. He majored in mathematics -- not business, not computer science.

Insiders tell stories about how he often knows more about the performance of their business than they do, and he isn't afraid to dive deep. Once when a new executive didn't know some detail about customer numbers for a product, Ballmer corrected him. In public. That executive was gone a year later.

Ballmer may appear irrational from the outside, but his decisions are driven by the numbers. Sales. Revenue. Expenses. Income. Trends. That's it.

There's been a lot of outside speculation about what Ballmer was trying to signal with his public dismissal of Server and Tools president Bob Muglia on Monday. Muglia is universally considered one of the good guys -- one employee said he found the news devastating -- and his business was great, with revenues up 50% and income doubling in the last five years since he took over.

Was this an assertion of control? Given Ballmer's intense focus on the numbers, the decision was almost certainly NOT driven by emotion or some desire to show the outside world he's in control. It was probably driven by some trend or sales figure that he found alarming, and that he felt needed to be corrected.

Ballmer has been very firmly in control ever since resolving some early disputes with Bill Gates, and especially since Gates left full-time duties in 2008. Nobody within Microsoft doubts it, and nobody outside the company should either.

We hear that the two had several discussions and finally reached a place where they just couldn't agree.

A former Windows employee suggested that Muglia and Ballmer disagreed over the direction of Windows -- perhaps the recent decision to support systems on a chip (SOCs) and the ARM processor, which could cause complications as Muglia's group strives to release new versions of Windows Server every two years.

All this is speculation.

For what it's worth, employees who work closely with him say that stories about his temper are way overblown, and he's actually fair and generous -- and he works like a maniac. If you disagree on something fundamental, though, he's still the boss.

That sounds like just about every other boss in the world.