Do you mean that the sound varies when you turn it on, or after you have been playing it for a few minutes?
I mean, if you have been playing it for a half an hour and say to yourself; "Why does it sound better now than it did last time I turned it on?", that would obviously be because the tubes have been given time to warm up. Maybe last time you only had it on for a few minutes. Know what I mean?
Otherwise, I think it is one of those weird cases of your ears playing tricks on you. I have moved my amps all over my apartment trying to get the best acoustics. I always think it sounds better in a different corner for a few weeks and than find myself switching it to a different corner. It 's just one of those weird things. My wife will come in and say "Did you move your amp again?" My response will be of course, "It sounds better in that corner." She will say, "Uhh, no it doesn't, it sounds exactly the same."
Bless her heart. :D