Was this after a few hours when you measured or immediately right after?
Hm. Keep this up for a 2 weeks to see if there is even a larger fluctuation for the future sets.
This is a combo of blood true expansion and partial lymphatic fluid retention. Allow an hour in before you measure. This allows some of the fluid to leave, and you'll have a closer to true expansion measurement.
You are following @kriskros methods as well. Just make sure to offer yourself quite a bit of protection as you use any hard clamping method. Soft restrictive is not a problem.
Thanks.
For post session measures, i always measured right away when i finish the session as im already doing PE. And don't want to deal with it later on.
Some times as a check i do measure after a while or next morning.
Will keep it up and update.