Yeah, that happens. What mod you using?
Your coils are rated to 0.25, but that's only in theory... like anything else, it's an imperfect device. Just like you can't expect a car to get exactly the MPG it's rated for all the time, you can't expect every coil to be exactly 0.25.
As for the changes - to me, that's very minor. Hundredths of an ohm are very tiny changes in resistance, and your mod is probably responding to that as you vape. Two things though:
#1: make sure your coil is firmly in the tank and your connection from the tank to the mod is good. you don't have to ratchet the thing in there, but firm is good, loose is not

.
#2: Your device may have a "resistance lock" feature. This makes a mod regulate at the resistance the device reads once you engage the feature. Even if you don't, it's not a huge deal. The level of resistance change you speak of is really minor, and not unsafe.