Bownarra nailed it, but I want to address some of your other questions too.
"I'm thinking that the reflex gain is compensating for the amount of wood I'm taking off. Does that sound logical?"
Basically yes.
"When I bend the belly wood and glue the boo to it and take it off the caul the belly tries to straighten out. The boo stops it but not until the back and belly are in equilbrium. The back stretches and the belly compresses."
Well not really. At this point the belly is under tension. At least as relevant as the back and belly simply 'opposing one another' are the materials' relationships to the glue line. Once off the form, they find a temporary 'equilibrium', though there will always be strain between them. As you remove belly wood, you begin to give some advantage to the other side, and the bow blank gains reflex. So you remove more... and it gains more... so you remove more... and so on. Then sometimes, all of a sudden it seems, it comes around and you're under weight.
Prestressed bows are inherently stronger for their thickness than other types of bows. Even though it can gain reflex as we remove belly material, are we removing some prestressed properties and inherent strengths? Does this have anything to do with why 'all of a sudden' it comes around, and we struggle to make weight afterwards?
Pulling to target weight from the beginning of the tillering process isn't the solution with these. Say we pull to target weight and it hits it very early. That means we glued a bow into reflex with too much wood on the belly, and removing it is what can cause this thing we're talking about.
When I glue up a bamboo backed bow, trilam, etc... I DON'T want it able to pull to target weight early in the draw. But I'm not just going to yank it all the way down until I hit target weight either. I'm going to shape, tiller, and exercise it along the way. If I did well in advance, with minimal wood removal, I'll hit target weight farther down the tree.
Like Bownarra said, the key to eliminating, or minimizing, this 'phenomenon', and I'd add, to make this a more predictable, straightforward, and successful endeavor, is to have your material closer to finished thickness, and taper, prior to glue up.
I grind belly wood for backed bows, trilams and such the same way I grind lams for glass bows... in my thickness sander, often on the lam sled, aiming for just over finished thickness, always with at least one piece tapered in advance too.... it allows them to bend easier into the glue-up profile, helps eliminate the reflex gaining you're talking about, allows me to more quickly and easily hit target weight and with good tiller early. With these bows, the bulk of the work is best done as prep work. After glue up, it should be gravy.
Leave yourself a little extra wood, but as little as you can safely get away with.
