A: There are several things that could make your amp sound different to you, including AC wall power variations and fluctuations and the humidity – both its effect on sound traveling through the air and on a paper speaker cone. Your guitar’s tone may also vary slightly due to humidity changes, aging strings, and so on.
But the biggest thing is probably your changing perception – everything from your mood to what you have been hearing/listening to prior to turning on your amp may affect how you perceive the tone. Plus, your hearing can vary from day to day, depending on a number of factors, including subtle ear fatigue.
There’s not much you can do about these variables, although it’s not a bad idea to put your amp on a voltage regulator/AC filter, and you could try to strictly control the humidity in the room, though this isn’t easy.