Sometimes collusion may be involved. Right now Foundry + antpool is just over 50% of network. So on paper they are separate pools, but who knows if there's communication.
Discussion
They're just pools, not miners. Miners are the ones who would be colluding or communicating. And I wouldn't be surprised if there are some large miners who mine on different pools at the same time, just to avoid the pool getting close to 51%