If I remember correctly, weren't British carriers of the era a little bit heavier, as they preferred to put more armor on them, while American carriers were a bit lighter, due to having less armor? I could be wrong of course, but that was my general impression- and a change in priorities could lead to the tonnage offset being found that way.
In addition, there is the loophole, if you will, of experimental ships- the USS Langley and IIRC USS Ranger did not count against total American tonnage as they were not designated fleet units.
EDIT: And to clarify, I was talking about someone thinking up the idea in the 30s, not necessarily of it hitting the production lines then.