The pathologists said that the rigor indicated a time of death something like 9 p.m. to 2 a.m. Then when they added the putrefaction (smell on the body) noticeable by 1:30 p.m. when the body was found it pushed it back to 10 p.m; the earliest time allowed by the rigor. It's all an estimate based on body weight, estimated room temp, etc. If the room was any colder than what they assumed it would have happened even earlier.
I think the key take-away from it is that, regardless of EXACTLY when she died, she more likely died when the parents were still up and about.
Well, one doubts after a long Christmas day that they were up until 2, unless there was some disturbance.
But it does seem like someone may have been watching the home to know right when they arrived and possibly even know when the parents went to bed. If they went to bed at 11, she could have been taken shortly thereafter.
However, the other scenario, of one of the parents messing with JonBenet once they arrived home and accidentally or not killing her and the other finding out, would explain the nightlong coverup and the Ramseys in their same clothing.