I *think* it's like this...
At a few (TRACON only?) fixes which have published crossing restrictions with a track record of deviations, the computer is set to flag and collect possible deviations. This info goes to management (ATC? FSDO?) which eventually decides what to do with it. The controller may not be aware that the computer flagged it. TRACON radar is often more accurate than en-route...they can always see your Alt (mode C) but en-route it may be hard to see on radar exactly where you were relative to the fix.
Controllers *typically* only report a deviation...
1) If it was grossly unsafe and/or created an obvious conflict. Even then maybe just a phone number and butt chewing.
2) Caused the controller's own alarms to go off...now he's on the hook so he'll understandably want to shift blame if the pilot was at fault.
3) He has a bad attitude or in a bad mood...a little kindness on the radio is cheap insurance, plus it make's everybody's day more pleasant.