It's because e.g.

a) Its of lower importance to for instance the work distribution... cant have crunchers waiting on new jobs to keep their cpu/gpu busy. :D
b) Validators are paused at times for scheduled other tasks. Most often this is in the morning session.

The principle being that some tasks run better when they have the server to themselves [a ltd resource], rather than multiple processes [daemons] trying to access the same records/datasets at the same time.

Since WCG only runs the My Grid statistics update 2 times a day, processes are tuned to make sure, particularly at 00:00 UTC, to be fully up to date with validations for any task that is quorum complete / has reached quorum minimum. The midday and end of day statistics determine the rankings, if rankings has your interest. It's like a bank balance of a company. Interest is only paid on what was there at midnight, not what was there at 13:43:27 for a fleeting second.