It's not so much that they vary by form set, as that they vary by era. Kukkiwon/WTF has specific international standards. The current standard is for stances to be more high and narrow, like what he's describing for the Taegeuk forms, which are the current forms of the organization. The old way of doing stances were deeper, wider, more karate-like stances. That's how I originally learned the Taegeuk forms, and I guess how the Palgwe forms were taught (I've never learned them). From what I understand, which could be wrong, the Palgwe forms were discontinued by KKW before they made the switch to higher stances, so when the Palgwe forms are still taught it's with the old stances.