>Certainly this can't be how it is designed to work.

I suspect that's pretty much the situation.  But being generous designing a pedal to act as both a decent wah and volume control must be quite tricky.  Wah pedals typically have different types of taper than volume pedals.  There's a decent engineering challenge there in overcoming the fact that they are fundamentally different beasts...  I'm no sure how the crybaby with combined volume and wah does it or whether that works any better.  It seems to have patchy reviews.

Maybe a higher end device would work optically and emulate the different response curves in software.  Ultimatley someone had to make a decision between building an uber-complex high end pedal versus building a more compromised one on a budget.

It's a shame that none of the reviewers seem to have mentioned this though.