RDA. Why does one coil gunk up way faster then the other?

I’ve had this happen before but never so uneven. I’m posting a pic l. Idk if you can see it weel because my camera sucks. One coil looks almost brand new. Left. The othe lr is almost completely black. Right. They seem to be burning evenly and I know they were heating evenly when I first put the build and removed the hotspots. Anyone know what causes this? They are identical coils.