Multi-parcel rates - only one courier returned (BUG?)

Using the API docs for Rate Request (https://developers.easyship.com/reference/rates_request), I've noticed that if I add more than one parcel to the payload the response only returns one courier (UPS). If I have only one parcel, then I get many couriers returned in the response (CanPar, UPS, Canada Post, Purolator… etc).

I'm wondering if this is a bug, because I'm pretty sure some of these couriers offer multi-parcel shipments, so why doesn't this work for rates. Any ideas? Suggestions?

I'm using my production key for this, as the sandbox key isn't very reliable for returning consistent results.

Thanks for the help. :)