It's not just producing a legal line for each line-bidder at, or above, the g-line. That is one constraint that the solver must meet but there are others. The solver must also meet other requirements such as the amount of unassigned flying and daily reserve coverage.
With traditional line-bidding there are similar constraints but they are applied to the lines prior to the bidding process. In that process you don't get to ask for things that aren't already built into the lines. You have to hope that a line that you can hold already has what you want. With PBS you get to ask, and you can often get, attributes that would never have been built into hard lines without PBS.
The way to avoid this is to have more Bid Groups which systematically relax your constraints so that you can get as much of what you want as your seniority will allow.