Robert_Charlton - 5:59 am on Apr 20, 2012 (gmt 0)

however I am now considering this a manual filtering

HuskyPup - My answer essentially assumed a link filtering of some kind. Note that I'd said...

If your backlinks for the US spelling were discounted for whatever reasons, that could explain the US spelling rankings drop.

What I was addressing was your assertion that because some queries were affected differently than others (in the way that you had described), Google was screwed up.

I don't know the history of your link-building, but I think the results suggest backlink problems... perhaps too many exact anchor text matches on the American spelling (keyword2), which is what your efforts were probably targeting. Obviously, it's hard for me to say without knowing many more specifics.

keyword1 keyword2 keyword3, I'm there at #4

Again, this seems to make total sense. It appears that keyword2 is dragging you down... perhaps because of anchor text penalization or the discounting of the backlinks with exact matches... but the presence of keyword3 (apparently not filtered because it hadn't been pushed too hard in link-building and thus hadn't gotten penalized) is helping you. Just a guess.