When I picked up mine it was like that, but the rears were trying to catch up as best they could! No idea why they are like that. When sanding the front rims I was surprised how much white was still under there.
It has been discussed before and I think the best explanation is different vendor with a less than stellar paint process. Sadly, all of my riders suffer from Front Wheel Discoloritis. The first thing I'm doing on the next one I build is get them off, blast them, new bearings and races, and then start working on the tractor! I always get them running but forget to do the front wheels.
That is interesting. I also found that most of the "discoloritis" was clay, dirt, grease, etc. Lots of white paint under there. Took degreaser, scrubby sponge, sandpaper and a pressure washer to get that **** off. Lots of elbow grease from my 11 year old as well!
Outside of wheels was less rusty than the inside.
I agree with Jimmy that just starting with the front wheels is probably the only way to reliably get them done!
I have one other theory: size.
The smaller front wheels and smaller tires simply put them down in the dirt more.
Especially if one is running front-mounted implements. The fronts get first pass at whatever the implement cut, tilled, churned, etc.