If the ColorFunction is of the form ColorData[...] it processes it differently from generalized functions. They are equivalent in general, but GeoRegionValuePlot treats them differently.

Yes, I assume those plateaus are reporting errors of some kind. There are also some negative ones, which are even more suspect.

About the ColorFunction, you also might think that

ColorData[{"RedBlueTones","Reversed"}][#]&

is equivalent to

ColorData[{"RedBlueTones","Reversed"}]

but GeoRegionValuePlot will bin the data if you use the second, but not if you use the first. I think it is much more useful to look at the unbinned data, so I often wrote this weird ColorData[...][#]& construct. I believe there is going to be an option to get around this in a future version.