I think subject2load is correct. On one occasion when both showers were in use I got up to go to the loo at the back of the cabin just as a pax from J came through the curtain and into the loo. A crew member also saw this and suggested I return to my seat saying that she would lock the loo when the occupant left and come and fetch me which she did.

The occupied/vacant signs in J clearly indicate that the loos are at the back of the cabin while those in F indicate the showers at the front and the loo at the back of the cabin. I think that HavocIOG is spot on and that some (probably regular) J pax are just trying it on. I suppose for most of the time this doesn't really impact on passengers in F although perhaps some may consider that the extra which they pay entitles them to ready access to "facilities" without having to queue unnecessarily.