Depend of avakening,mood,refine,stars of the ninjas.For example,if you replace 4 stars ninja with 3 stars one,power will go down.

Also your gear will be moved around, so you'd have to give gear to appropriate ninjas too

assist link ninjas do not cross over to your new team positions (only stays on same ninja), group summon level lowers can cause it, star levels, and + some ninjas have different stat gains so some can get MORE TAI each time they (level) and some can get more NIN each time they (level)

every ninjas has their own BP like the level of mood in each ninjas, bond skills, probably in treasured tools, stars level etc. those individual power affects the whole BP of a team that is why it keeps on changing every time you change ninjas.