As DrHowser said, the rows do not line up. There are 4 rows on the right side (EF) and 3 rows on the left side (AB). So, as you're walking towards the back of the plane, you pass 3EF first, then 3AB, then 4EF, etc. until 6EF which is the last row.

Row 3 has much smaller overhead bins, which is patently stupid since the folks there have to use them instead of the seat in front of them. I personally like 5E the best on the MD80s, although on ORD-LAS westbound I'd shoot for 4E to get a better shot at my choice of meal.