an unfortunate fold

[Canuck]

There could be three things that are causing the pucker of your front apron.

The first is the right hip strap too tight. But you said you only have two straps so that's not it.

Actually Steve I have to disagree with you here. It can happen when the right top strap or single right strap is too tight. By snugging up the under apron of the kilt is what actually holds the kilt up. The over apron can be attached without any stress on the strap or front apron.
I agree with your other observations.

[The Wizard of BC]

Oh Robert, I think I understand what your saying is uneven tension on the two upper straps? I never even considered that someone would intentionally do that. And you are correct. if the right top strap is carrying all the stress of holding the kilt in place there would develop a diagonal stress pucker across the apron. Duh!