Back in 2012 I flew from Tenerife South airport to London Stansted with Ryanair airline. All the tickets for this flight were sold out. But 3rd and 4th row on this Boeing 737-800 were empty (12 empty seats) 1 & 2 row are for customers who paid for the seats. The interesting think that no one was allowed to seat there. One lady before departure occupied one of the empty seat in the 4th row. Cabin Crews told her that aircraft will not depart unless she will go back to her seat. At first lady refused to go back and started complaining. We were waiting for her to change the seat and then we departed. I did ask the steward why is there two empty rows but had no clear explanation from him
Were these emergency exit rows?
Seat pitch on low cost airlines is too small to allow an emergency evacuation within the required 90 seconds with regular seat pitch in the exit rows. Therefore, the seat pitch is larger there, which makes a world of a difference for tall people. Low cost airlines have decided to monetize those few more inches of leg room and charge extra for sitting there.
If no passenger on that flight was willing to pay the extra fee, those airlines which pride themselves on treating their passengers with the utmost contempt keep these emergency exit row seats free even if the plane is fully booked on all other seats.