I think over the winter bats definitely feel heavier especially in the UK but I think a lot of it is just in our head and also based on form. Yeah, I think your strength changing can definitely affect it too, I noticed my Kookaburra felt lighter since I am still growing but then again maybe its just in my head
Put your bats on the scales. They could have gained some moisture in the winter.Or you could have to do some work in the gym...Bearing in mind there has been a pretty wet winter and we have been in lockdown, I am expecting both.
Is your question how often do bats change weight based on moisture levels or how often would you expect your muscular strength to change enough to effect your perception of the weight of the bat?
I also find that storing bats in a garage/environment with more humidity, the willow fibers can expand and roughen the surface finish a little.
It's totally possible that 1 bat has gained moisture (and therefore weight), and 1 bat has lost moisture. Depends on the humidity of the room they were in and the moisture content of the willow before you left them.