It depends on the video surveillance system. Many of them just record into computer and will use the time from the computer. Unless the computer is synced to the "actual" time from time to time via a network, they will "drift" based on their internal clock. I have a computer in my lab that will drift 30 minutes in a matter of months. It's not hooked up to a network, so there is no way for it to obtain a referenced time.
When LE is reviewing video several days (maybe weeks) later there is no real good way to determine if the time stamp is completely accurate. Their best guess would be to compare current time on the computer or video system with the actual time. Then make some broad assumptions that the error found in the present would have been the same during the video segment in question in the past.
Since LE seems to be emphatic that LS was seen at 2:51 AM, I am guessing that surveillance system must have it's time verified. Maybe the 2 systems used on the "white truck" were not synced as well. This may have lead LE to think that it passed thru a couple of times when now it seems it only passed thru once. Just my opinion.