Dear Reddit, why does the water temperature change when switching the tub to the shower?

I noticed the water gets either, colder or hotter when switching it from the tub faucet to the showerhead, but only when you first turn on the water. Doesn't seem to happen after your water's well adjusted. Is it just my shower or is this a normal thing for everyone?

P.S. My shower isn't super fancy. Just your basic 3 knob shower.