Members gain leverage they would not have individually. The group earns tier rates on aggregated volume and allocates the proceeds back according to what each member reported.
The systematic leak is under-reporting. Purchase lines that arrive uncategorised, arrive late, or fail to map into the group’s product taxonomy simply do not count — and because the group settles on what it received rather than what it should have, neither side detects the gap. Five to ten percent under-reporting is common.
The only reliable control is a three-way reconciliation the member performs: purchases, versus submissions, versus what the group settled.