Actually, dools is quite correct. Generalizing from spelling mistakes is a classic technique for tracing phonetic shifts in language. That's because misspelling is almost always in favor of something that sounds the same (e.g. their/there/they're).
One couldn't ask for a better example than "then/than". When I read the previous sentence out loud, the first "than" sounds closer to "then" than to the second "than".
Unlike us Commonwegians, (some) Americans pronounce them identically. Hence they're more likely to interchange them in writing, either if they really can't spell or if they're half-asleep.
