Body Battery difference between Vivoactive 4 and Fenix 7

Hey all, after upgrading from Vivoactive 4 to Fenix 7 I noticed there are huge differences between the stress levels and body battery. Wit the Fenix I'm almost never reaching 100% body battery, it's mostly maximum around 80% if I sleep well like 8 hours. Whereas with the Vivoactive I reach almost every day 100%. It seems like my stress level is overall higher with the Fenix. For example with the Fenix I almost never can increase my body battery when I'm relaxing during the day whereas with the Vivoactive already a short rest increases the battery level quite easy.

My question is if this is really an issue of the different hardware or are there maybe different settings which might influence the body battery? Or is one of the watches maybe not working correctly? How often do you reach 100% of body battery in average?