ISV Cleaning

[Brice Warnick]

Kneales post gave me something to do today for cheap.  Auto Zone has
carb cleaner for 99¢ a can. So I pulled the ISV and sprayed it out
real good. It is the original ISV and to my surprise it was extremely
clean. I sprayed it anyway since I had it off. I took a thin flat
screwdriver and opened the inlet  and took another thin screwdriver to
hold it open and the sprayed a bunch of carb cleaner in and capped each
end and shook it real good for awhile. No residue at all. 
Dumb question but why are some ISVs really clogged, some medium dirty
and some really clean? What causes that?