I'm guessing because it has more to do with the establishment of his sound throughout his discography or something like that?
I might be wrong but its all that comes to mind
That would be my thought.
I'd be interested to hear Andy's thoughts on the question and whether he would really think he has "a signature sound"? or is everything really driven by the individual song, album, desired outcome, ...
It is an interesting question, one I'm probably not qualified to provide input to as I readily admit that while I appreciate the production side of music and have worked hard to understand it as much as I can and apply things I like to my own music, I don't really look for signature sounds in music, it really is as simple as I like this, I don't like that, If I can enjoy a song, or a band, the production, while important and surely playing a part in that enjoyment on some physical level, is secondary to me (I know, I know - possibly sacrilege to some around here but I'm being honest).
There are very few albums I would say I enjoy simply because of the production - for me a great production does not make bad or boring music tolerable, but like everything else, it's simply a personal opinion. But then again everyone's taste in music and how/why, they enjoy certain aspects is very much an individual thing.
Back on topic however, I'd really be interested to hear Andy's thought's on the specific subject and the larger question of signature sounds in production, not necessarily the music itself.