Continuity was never Rob Grant and/or Doug Naylor's strong point. For example, the ship's crew numbers 169 in series 1, but later becomes 1169 (after the crew died...). Lister also had his appendix out twice, and his and Kochanski's relationship also varies wildly - in the first series, he says they never had sex, and Holly says they shared a total of 173 words. But later on, it's shown they were together for several weeks and had a lot of sex.
Basically... don't worry about it too much. It's all part of the same continuity, Doug Naylor just doesn't bother much with the details. If you watch through to the last episode of series 10, he even takes the piss out of this by dangling the prospect of us finding out how exactly everything did happen.