Bluesrains

Ive been using the Kanger ProTanks a few months now. I use them with a Vamo Mod set at 4.0-4.2ohms and my Protank coils are 2.5ohms. After a day, I notice the coil on my ProTank reading higher ohms, 2.6-3.0. Why is this happening?? Isnt it supposed to stay at 2.5ohms? ---- I use a juice mix of 50/50.