This is primarily for US sellers …
Pirate Ship has moved to PM Commercial Plus pricing. If you download Notice-123 from USPS, you can see that PM Commercial Base and PM Commercial Plus are both showing the same prices.
If you download the pricing schedule from Pirate Ship, you will see that some zones/weights have a green tinted background. Those cells are eligible for a lower price (up to $1.02 below what is shown).
My best interpretation is that USPS is giving some flexible pricing incentives during periods of lower package volume, but taking it away when things ramp up. I’m not even sure if Pirate Ship knows exactly when this happens, only the maximum incentive that could be offered.