i wish i could offer some chemistry input into this discussion, but i would like to offer my simpleton logic.
could it be something to do with the fact that the bottom is sitting in the solution for longer, soaking it up, infusing with it, or whatever is happening there?
the top evaporates pretty fast, while the bottom takes much longer.

I'm not sure I understand.
A solution is homogeneous, so there is as much goodies at the top as at the bottom as long as we're in the solution phase; if the dregs are more potent it's because the DMT in them is undissolved. And evaporation has nothing to do with it; we're talking about what happens after it settles.

A more precise data point:
I ate as little as 120 mg of dried dregs and I felt it. Not strongly, but it was most definitely active. I'd estimate the DMT content of the dregs to have been between 10-20%.

i re-read the OP and i completely misunderstood the question
although it looks like Elrik answered it quite confidently (and seems to know his/her stuff from other posts around here too).

Assuming insoluble DMT tannate is trapped in the solids, could you recover it into a liquid form without an A/B extraction?

For example, by adding a stronger acid, then evaporating it?
E.g. adding HCl to displace the tannate anion, filtering out the solids, and driving the HCl out / evapping it to water-soluble solids? (evaporation may have the caveat of the water evapping faster, raising the HCl concentration to 20% and potentially destroying the molecule).

Also, doesn't the fact the solids come from concocting plant matter mean they're soluble at boiling temp and at the concentration at the time of the boiling?
I guess not completely, because solids get kicked out of the plant matter too, and the wash being clear doesn't mean they're not there.