Many people can not or do not select seats before they check-in (or later). And the sales availability is that: how many they are willing to sell.

There may be 108 seats unassigned, and them willing to sell 9 more tickets, but it be overbooked. (Though unlikely).

If you want comfort, pay for it. Cash or eUps.

The tea leaves here are a fun game, but no guarantee.

Edit: I thought you had counted the unassigned seats from the map, not totalled the buckets. Either way, my point stands.