The original game had a very arcane spelling. Most of the seemingly misspelled words are not spelled like US or UK English. Defence is a good example, as both US and UK spell it 'defense'. I'm not sure where the spelling came from in the first game, but it seems possible to me that it was either a different English dialect or that these are genuine spelling errors due to the lack of a proper editing person on the team.
The original game release was titled UFO: Enemy Unknown, and the US release was titled X-COM: UFO Defence.