I had a similar situation two days ago. Booking a flight YEG to YVR. Logged in, made my booking and first available seat was row 19. I found it hard to believe that rows 12 to 18 were completely spoken for 6 weeks out on this route, but sometimes it happens. Also checked to make sure that I was logged in as I know that lots of time these seats showed blocked for non E & SEs. So I took the aisle in row 19. When I got my itinerary minutes later by email lo and behold I had a middle seat in row 27. So I logged back into my account and went to change the seat. When I did, row 12 was completely occupied, but the seat map showed only one person in rows 13 to 19. So I chose an aisle in row 14 and got a new emailed itinerary that was correct. So, if it happens again, try going in through online booking and changing your seat yourself.