What would cause this??

[SemiSweet]

Mine developed those frosted bands almost immediately after I replaced my cam and followers and I suspect it’s from fine dust during install. Although, I had it apart recently to replace the rocker shafts and it seems like they are disappearing maybe since I've been doing frequent oil changes and adding gear lube. As long as you can’t feel anything with your fingernail it’s nothing to worry about. I always use compressed air to blow the edges of the valve cover before removing , those followers are very close to the edge of the head and dirt pretty much just falls right onto them if it’s not cleaned up.