Why the first five rows in Aer Lingus A320 are unavailable during online check-in?

I traveled with Aer Lingus (A320) many times and I always booked my seats during the online check-in (30 hours before flight). Although I opened the seat selection just after the online check-in starting time, seats in rows 1-5 were always unavailable. Maybe they are available later, or why they are unavailable?
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