Perhaps its because people say 'straight ahead on the greens' and then you look up and there's a whole block which doesn't have any greens in it?
Perhaps its because you can't see the greens on a bend and your lighting operators aren't quite as quick as you'd like to think?
Perhaps it's because red stop bars often apear at random due to other traffic interacting with the lighting plan?
I 'follow the greens' regularly at LHR and about 50% of the time I either have to slow the aircraft down to wait for the lights to catch up or ask for clarification.