Hi
The trigger is doing as you ask and is submitting all bets at a uniform price which is
(number of qualifying runners + 2).
However, Betfair will match bets at the best available price. For example, in your screenshot, if you look at the last selection in your bet list, the bet price is 13.0, but as soon as the bet is submitted, it will be matched as follows:
2.00 at 14.0
3.00 at 18.5
4.00 at 20.0
leaving 1.00 unmatched or matched between 13.0 and 13.5 (can't see the prices to tell)
Hope that makes sense.
perhaps also consider Dutch Back (action 23 in the manual)
or Dutch Lay the unqualifying selections