777s I;m not so surprised as the HDs were designed for Hub to Hub transport (as well as a few outliers), sending the Polaris metal seems weird, but also makes sense as they could be cycling in n out of hubs.

The A321neo though, from what I've seen on flight radar, it seems that ORD-SFO is now its most frequented route, I've filtered them out before and I've seen up to 4 flying to/from there.