Why are the two front wheels rims getting dirtier then the rear wheel rims?

What is your question?

My R350 needs a wash, but what's got me baffled is that the front wheel rims are a LOt dirtier then the rear wheel rims; my husband is concerned that it may have something to do with the brakes or brake fluid. We are still under warranty, so will take it in, but hoping I can get some good advice here first.