Pretty much every plot inconsistency in Red Dwarf can be attributed to the trope "Rule of Funny", e.g. where the comedy value of a scene takes immediate priority over any consideration of continuity or common sense.
This interview with Douglas Naylor illustrates this point quite nicely:
Do you think of Red Dwarf as a science-fiction show with comedy in, or a sitcom with science fiction in?
I think... well, actually, you could argue, it's been both! Whether we intended that is something else...
It was always intended to be a comedy first. They were really so linked, but we had to pretend it wasn't science fiction when we sold it, because everyone said that science fiction didn't sell... and yet the science fiction part, from our point of view, was actually our pitch! Because otherwise, it was just about two guys in a spaceship. And especially with nobody else there, what was it going to be about? Just arguments about chicken soup dispensers, and skutters. So it was the science fiction tropes that gave it the impetus and the energy, and the originality. And also, the ability to go anywhere, get off the ship, whatever.
But I think if it's a science fiction show with comedy, that's probably when we've done it wrong. It should be the other way around
Did you find it was more important to tell the best story possible at the time, rather than being concerned about whether it lined up with past backstory?
Absolutely, yeah. And in the end, the laugh is king - and Rob's feeling was certainly that if it works, it works, and don't worry about it.