The amount of money that a record label is willing to spend on a band changes with the amount of sales a band has. If a band sells more albums and has a big fan base, they get more money alotted for studio time, promotion and so on, and also, a good band gets to use better equipment that is more expensive. Maybe what you're hearing is a difference between the microphone in one studio vs another one when Johan...eh...growls, for example. There is a huge difference in the sound quality from the very first CD (from a million years ago) to the last AA release, WOOOS, and the band's sound must then be adjusted accordingly. I don't know if any of the instruments have been traded in for better models, but that could also make a difference in the sound. There is a big difference in the sound depending on the producer, and this band has been through a few of them. Then, of course, the sound of the band evolves as the minds of the band members evolve in various ways. The lyrics are often metaphors for events that the members have lived through, so obviously, the music changes with their lives. All of this means that you'll get an AA sound with some variety to it. You can't be too static in our song writing,either, or everything would come out sounding the same. I hate bands that put out album after album, and when you sit down and compare them, it turns out that you can actually sing the lyrics from any song to the sound of any given track of all the albums. AA doesn't do that. All the songs are distinct.
This question is really better off getting answered by the band members themselves, but for what it's worth, that's my input.