An idea, irrespective of how many of these tiles a kingdom gets, clearing them all, will be dependant, on how active and proactive the players in that kingdom are.
With particular reference to the level 8 tiles, some of the big players who probably have a very high rated watch tower can pretty much see the whole kingdom? Perhaps those in the know will divulge the exact details.
Given this as true, they don't need to do a lot of physical searching, they can just teleport where ever tiles maybe, as found by the watch tower..
If the L.8 tiles don't not empty much faster than the level 7 tiles in the forest, where most of the big players sit,because the Uber and level 6 tiles invaders and ghosts are concentrated there, it may not be worth their while to teleport to more isolated tiles to clear them.
Another point,I have heard it said, after a while you either do not need to yield,or it gets boring, I would imagine that every high level account or clan has to wonder what to do with all the influx of resources, and their production shut down,especially if they are either already researching and building or with none to do.
If a kingdom wins the fury battle it probably means they are a very active kingdom, and again you would expect a lot of level 9 tiles nearer the centre of the map, where most of the big accounts are found.
I am not sure if a more wide spread distribution of level 8 tiles is indicative of anything, but there was,probably still is, one right on the edge of the map.
It is.
I made so many spelling errors before,that went unnoticed by me, and the spell check?