Which is especially odd because the author (Sam Hughes) lives in the UK and wrote the original in UK English, but apparently wrote the rewrite in US English. For example, a chapter in the original was titled "Case Colourless Green", but in the US edition of the rewrite that chapter is "Case Colorless Green" (without the 'u'). So Hughes, a native UK English speaker, wrote the rewrite in a non-native (to him) dialect, then had it (lazily) translated into his native dialect.
It was probably to deal with the transposition [0] out of the SCP universe into a new one. SCP is vaguely 'set' in the US because that's how a majority of the contributors naturally write and spell things, which sets the voice of the world indirectly.
[0] AKA "filing the serial numbers off" when it's explicitly fanfiction instead of a shared universe model like SCP