@netmeg and all -- Interesting that three of those four sizes are "Recommended" ad sizes per Google and the fourth (468 x 60) is probably the most used size for tablet display. Also, in line with wa desert rat's observations that most of those sizes aren't practical on a mobile site or are at least very obtrusive in that environment, I'm guessing that most of this is happening on the desktop or tablet.
These are also fixed sizes. I use responsive ads almost exclusively (smart sizing) and don't see the degree of this problem that some do. I ask if this could be a problem with very particular units?
I had to just check this before making this statement because I don't use fixed ad sizes at all. So, I just generated a 728x90 banner to see the actual ad code to make sure. Sure enough, the following is in the code itself.
If bots are involved, this would be a key way of identifying and targeting a particular ad unit size. A worthy experiment (if you are able) would be to replace one of these fixed units with a responsive unit. Might take a bit of container div magic to get it to display in exactly the same size as the fixed unit but it can be done. Pick a troublesome unit and give it a try. What have you got to lose?
p.s. You can force smart-sized units to a particular orientation by changing the "data-ad-format" parameter value in the ad code like this...
For a square unit
For a horizontal unit
for a vertical unit
Add some container div dimensions and you're able to constrain the unit to the size and orientation you're looking for.