It’s sort of like asking MyFitnessPal how much you weigh by inputting everything you ate. It depends on what your starting weight was. It’s the same with Xert; it depends on what your starting signature was. A recalculation by default uses your current signature and assumes it’s your starting signature. In the same way, if you insert or add rides before your first ride, things can change.
Xert uses signature decay by default to help correct things but if you do a lot of easy riding, then there isn’t a lot of information to help hone in on a precise signature. In these cases, having a good starting signature and/or using the No Decay algorithm for periods where there aren’t breakthrough rides is the best way to set things up.