Wayne,
I'd agree with you if we were talking a few seconds of boilover here, but the OP is stating he lost HALF his volume to boilover. An event that takes 1/2 of your wort volume IS NOT evaporating water, it's pretty much all wort. I've had short boilovers before, and it might be foam, but it's sweet wort foam. You know when you clean it up.
The OP might be describing what happened incorrectly, but you don't lose 1/2 your volume to boil off, and your OG doesn't go up after a boilover, like all that dumped on the deck was water. Doesn't happen.
Your OG DEFINITELY doesn't go up from 1.064 to 1.1 after a boilover OR boil off AFTER you replace the lost boilover volume with water. The numbers don't make sense given the scenerio.