Random question of the day - why is there no junction 6 on the M27 motorway?

The planned junction 6 was simply never built. There was originally going to be a 'motorway spur' (a mini-motorway) coming off the M27 at junction 6 and going into the centre of Southampton. This spur would have been called the M273. They just never got round to building this spur so junction 6 wasn't built.