Current Microsoft employee here. A few years ago my manager at that time did exactly the above. The review process in use at that time rated employees on "What" you accomplished and "How" you accomplished it. I had done really well on the "What" and the manager still gave me a low rating because supposedly I did poorly on "The How". The "How" gave managers tremendous flexibility to rate employees however they want regardless of actual performance. So why did my manager do it? I believe he discriminated against me for age reasons. There is evidence to support this: This manager rewrote ("reinterpreted" according to him) the HR-supplied expectations for SDEs (Software Development Engineers) at each level, and he raised expectations for those at higher level, while leaving expectations for lower level employees the same.
While I believe this manager discriminated against me for age reasons, I don't believe there is Microsoft-wide, corporate-sanctioned, age discrimination going on. It might be happening at the level of individual managers, however.