I also assumed that it must be showing clicks for impressions.
If you look at your Amazon Associates reports you can look in more detail at things like what link types it's counting clicks on. For me, all the huge number of clicks are on the link types called "Basic Display - Detail Page." Interestingly, under impressions for this type of link it says N/A and Amazon doesn't track the impressions on this type of link unless you install a script to have it do so.
So I wonder if, since it doesn't track impressions on these, it's just counting them as clicks instead or something.
I'm also confused as to what these "Basic Display - Detail Page" links are. I think they may be the ones where you have a little graphic showing the product with price information under it. If so it would make sense. I have a bunch of those on one of my sites, several on some pages. So if it's just counting every load of one that could add up quickly.
This doesn't explain why my US Amazon affiliate account is not full of these massive numbers, while others are though.
How does this compare to the details of your situation, Nick?