https://github.com/10gen/mongo/blob/330458e81f9baa3f936d639dc3edbe1a8c4c5a32/src/mongo/db/operation_cpu_timer.cpp#L146
 
Not sure how often an operation's client can be detached from the thread, but in case it is detached/reattached more than once, the timer only stores the elapsed time until the last interruption.
 
Probably a matter of accumulating instead of assigning _elapsedBeforeInterrupted.