WinISD is going to spit out the "optimum" FR. Meaning as close to flat as it can get it. it's not concerned one little bit with gain. And that means it's usually a larger sealed side.
[Edit: wait... are you talking about the 6th order?]
WinISD will not give you "optimum" specs for a 6th. You have to input your desired values manually and it'll do the best it can to guess at the response.
Vf is the front chamber volume (volume front). The ported section of a 4th.
Vr is the rear chamber volume (volume rear). The sealed side of a 4th.
Fb is tuning.
Ff is the tuning for the fron tchamber of a 6th.
Fr is the tuning for the rear chamber for a 6th.
Toss in a 2:1 ratio for that 4th and watch that big ol' peak rise around tuning.
And convert that VAS to cubic feet so it's easier to relate. Lol.