I think you're pretty much right. I mean so far I've vaped 2 bowls and I think I'm a tiny bit high. I guess it depends on how many thc crystals there are on the stems. But since the buds are where the resin is produced it's not surprising that there isn't much on the stems and leaves.
edit: I'm vaping my third bowl and now I'm pretty high. It's not as good as vaping bud but if you're out and you really need some i recommend it if you have a vaporizer.