Playing in different rooms has a TREMENDOUS effect on your tone,sometimes good, sometimes bad. If you set up in a living room with plush couches and thick carpeting, you'll adjust your tone accordingly. Then, when you take out your gear to your buddies' basement or to a club, the sound will be totally different. All of a sudden, there's no cushions to absorb frequencies,mabey there'll be a higher ceiling and the sound will reflect all wierd. Mabey the place is wired in such a way that the outlet you're plugging into can't draw the same power as your house and the amp will suck accordingly. The lush ,deep Reverb you dialed in in your practice area sounds like **** in a bigger room with a ton of natural Reverb.
THEN throw in the variable of different combo's of Guitar/Effects/Amp...!!!
I play Telecasters,a very bare bones guitar. Now If I plug into my own setup (Tele-Wah-Rt-66-H2o-64 Deluxe ri), I'll get the sound and touch I'm used to. If I take that same setup and plug into my buddies Marshall, it'll be a different animal altogether.
As well, the same setup can sound different at different volumes. At home, the Amp sit's around 1. At a gig, I'll have it around 3-5,and it sounds a bit grindier (I think thats a word!!!), and full as the amp starts to breakup.
To all that, add the fact that gear sounds different when played with other instruments. When Bass ,Drums ,Keys,and other guitars enter the mix, some frequencies get lost. The Bass heavy tone that sounds amazing when your by yourself, sounds like a big electric fart with a group!!! A mega-distorted sound can sound like a mutant mosquito when the drums kick in!!!
Just some thoughts,
Ian