Imagine doing the accounting.
I'm sure they have a computer program set up for doing that.
Here's how anthologies usually pay their authors:
An editor pops up who sells an anthology idea to a publisher. The publisher says, "Great idea!" and gives the editor an advance. The editor uses the advance to buy stories.
The editor turns in the anthology, it's published, goes on sale, and garners royalties in the usual way (15% of the cover price of each copy sold; when more royalties have been collected than the original advance, the overage goes to the editor).
When/if the anthology has earned out the editor gets a biannual/quarterly/whatever check. The editor keeps half (reward for the work of putting together the anthology). If an agent was involved the agent's percentage comes out of the editor's portion.
The other 50% goes to the authors in one of two ways (and which it was will have been spelled out in painful detail in the contracts between the authors and the editor). One way is this: Each author get's one share per story. So in an anthology with 100 authors, each would get 1% of the 50% that would be the authors' share. The other way is that the shares are split up by story length, so that if an author's story was 3% of the anthology's word-count, that author would get 3% of the authors' 50%, while another author whose story was 0.5% of the anthology's word-count would get 0.5% of the authors' 50%.
Centum's plan seems to be designed to screw authors. Not from malice, I'm sure, but rather from ignorance.