Basic question about PPLNS, please see this pic (from Zcoin pool)
https://imgur.com/V0KQeFD

I submitted 100.4 valid shares, if by PPLNS, the next block reward is distributed to the last 156,938 shares (with current block reward 28 XZC), why is my block reward only 0.00056921 XZC and not (100.4/156,938)*28=0.0179 XZC?
Even if I didn't understand PPLNS and you would apply proportional distribution, I should get
(100.4/401,700)*28=0.006998 XZC?

Why is the block reward so low?