On the Dash there will only be 2-2 seating, so perhaps they block off a seat on each row for the first few to give as close to a business class service as possible.
That will be the most likely scenario. They might also reconfigure dedicated aircraft for those longer routes with those first few rows having a slightly better seat pitch.
In fact they quote their Q400s as having either 72 or 76 seats. From that I would deduce from that the 72 seaters have 2 or 3 rows with extra leg room.