These are the economy rows where VS often put special needs passengers. I would have thought that any unallocated seats in the mini-cabin that rows 21, 22 & 23 make up would have been available at Twilight check-in.
Given that there are only 27 or 30 seats [depending on aircraft configuration] in this mini-cabin, they could quite easily all have been allocated to parties travelling with special needs passengers. (Especially if you travel on a peak day, such as a weekend to a destination like MCO).
Someone here will doubtless have a more definitive answer though.
James