Let's try to guess the reason. Maybe most people who stand at standing desks simply:
1) just stand,
2) don't hang from a crossbar on breaks,
3) don't lie with their legs at elevated position on breaks,
4) don't necessarily move more than those who sit at their desks,
5) spend the rest of the day sitting anyway?
I, for example, stamp on my feet a lot, hang from a crossbar for 30-60 seconds at least every hour, lie with my legs on my couch on breaks for a few minutes and spend at least two hours a day walking my dog, not to mention regular workouts. And when I'm done I don't sit, generally: I watch movies and TV shows on my bed, read books mostly while lying on my carpet and rarely drive (and don't commute).
I also suspect that those who stand a lot at their desks tend to sit more after.