The most likely explanation is that every butt has its own cooking and plateau time. You just may hit a few longer time ones.
On the other hand, I have long suspected that radiant heat might affect the plateau time. I speculate that this is because radiant heat penetrates deeper than than the heat from convection. The BGE cooks a little faster than the WSM at the same temperatures and I believe this is the reason why.
I suppose it is possible that the new smoker radiates more heat than the WSM?
Jim - Another transplanted Texan
Former KCBS CBJ
Large and Medium Big Green Eggs , Black 18.5" WSM, Blue Weber Performer - Stainless, Green Weber OTG Kettle , Brinkmann SnP Pro, and a Stainless UDS. One retired Portable Kitchen grill.
Red Thermapen, Maverick ET-732, EdgePro Apex Sharpener.
Avatar is the original 1951 Weber Kettle