I really don't know but had the same thing happen to me. I'd had both bottles for a while and in the past both had produced a nice, bright, polished finish. Hadn't used either for a while and this spring I was spraying a P-38, finished one bottle and switched to the other and the second bottle gave a dull, darker gray finish. Same day, same brush, same pressure, same basecoat and it all went wrong.
Oh well, turned out I still had a few scratches to get rid of anyway...
Tony