Just be glad you don't have white wheels.
I recall the gloss black wheels on my M900 getting quite dirty. The 998's wheels are more of a dark silver, and don't show the grime as badly, but they also collect dirt.
Brake dust tends to collect on the wheels, and streets aren't clean places to begin with.
*shrugs*
(As to why your front wheel wouldn't be collecting dirt, I dunno... It's more of a pregnant dog to clean, though, so count your fortunes.)
