It's the nut on the handle unable to hold tight for all five. If you can't it's not the rifle it's the shooter that causes the groups to open up shooting five.
I do not think that is true at all. You could put the rifle in a vise and try to shoot five shot groups and you would still see variation.
The thing that really is at the root of groups opening up is variation.
The more shots you shoot, the more time the variables involved have a chance to exert their influence on group size.
The whole idea of process control is variable reduction. Shooting is a process and the more variables you can isolate and minimize those effects on the process, the better your groups will get.
Your bullets, primers, brass, powder and your technique all have variability and each one has a unique influence on your ability to put 5 shots into one hole.
The best BR shooters have done their best to reduce those influences, but they are still there.