In principle the flight can be oversold and wide open in all classes.
To see this, consider that the availability may be based on the demand to come.
If the optimiser sees that:
* there are 100 seats in total
* 105 bookings are held
* 10 passengers are expected to noshow
* based on the history of this flight no other passengers are likely to materialise.
So the optimiser will say to itself, "OK, I am expecting to have 5 empty seats on this flight and I don't think anybody else will be demanding a seat, so I might as well open all classes in case a low willingness to pay passenger comes along. If I do this I will not be risking displacing a high value passenger because I don't think any are out there. And if either a low or high value passenger pops up then I am increasing revenue."
Or something like that.