Winter coats molting..question

Hi guys! Was just asked this by a friend.."curious question that I thought you might be able to answer - why are my "rugged" greys malting furiously, but my "unrugged" & "rugged" chestnuts/blacks/whites aren't. I always assumed malting had to do with daylight hours - yet it is only the greys doing it? Just curious if there was something different in their makeup??"