It's not grammatical, it's just a pronunciation thing, common to British and (I think) Australian accents.
This leads me into one of those ridiculously nitpicky questions that has no possible answer within the series.
Everyone communicates via translation microbes, right? Only native words that don't have a direct translation - like names - would come through as they are. Those words would have to be learned through simple repetition and mimicry.
So why, if Crichton introduced himself as John, with an American accent (JAAN), does Aeryn pronounce his name with an Australian accent (JONN)? She should pronounce it as he pronounces it.