Well, my guess would be saturation. The first couple pulls are bound to be more saturated with DMT. When you base in an aqueous solution the DMT will rise to the top where the solvent can become easily saturated.
If you really want fluffy white crystals with ACRB you will need to do a mini a/b. That seems to be the most reliable method of separating the crystals from the goo.
Typically, 6 - 8 pulls. Although I've done upwards of 12 before. Depends on the bark. Yield decreases exponentially with further pulls. The more you pull, the more you exhaust the bark/solution.