There are many and varied reasons why seat numbers are not consecutive. In reality, when the safety brief is telling you to count the number of seats between you and the exit, it means physical seats, not the seat numbers. Think about it, if you are crawling through a smoke filled cabin, you're not going to be looking at the seat numbers that are way up at the lockers or on the PSU, are you? You're going to be on the floor, counting seat legs.