All three of the "legacy" airline online shopping portals are run by the same company: Cartera Commerce.
This is complete speculation, but I'd guess that Cartera goes out of their way to ensure that one airline isn't getting a better deal than the other two. Perhaps they accomplish that tenet by spreading around the miles per dollar spent so that each client has a few standout offers in each category.
If someone really cared, they could make a spreadsheet and compare offers across the three sites. That task might show a pattern or it could be a complete waste of time because this Izod example is an outlier.