In the LiveUpdate policy you have configured LiveUpdate to run daily with a randomization period of + - 12 hours (24 hour window) however you notice that all Symantec Endpoint Protection (SEP) clients are running the LiveUpdate task within a 12 hour window instead.
The 24 hour randomization window was replaced with a 12 hours randomization window in Endpoint Protection 12.1.5 and later.
The start time will be randomly selected from either the configured start time +12 hours or - 12 hours, depending on when the start time is configured:
To spread out the schedule over 24 hours the clients can be split into 2 client groups with different LiveUpdate policies applied: one to be scheduled before noon with randomization time +- 12 hrs and the other to be scheduled after noon with randomization time +- 12 hrs.