Board Thread:General Discussion/@comment-14250-20171008102723/@comment-30008142-20171008210356

As Ben explained, the Wattersons' surname is a reference to Bill Watterson, whose name is spelled with a double T. The correct spelling can be seen on-screen on many occasions; those are the few occurrences of the spelling I could recall:

Furthermore, the official Cartoon Network website spells it with a double T, as well as both CN and TAWOG's official YouTube channels. So, I think that's as official as it gets.

The single T spelling could be explained by the fact the people who work on the backgrounds and other various assets are not writers. It's possible some of them don't know the correct spelling or simply misspelled the name and it was overlooked. After all, the show staff are human beings (supposedly) who work on a very tight schedule.