I think the answer to your question lies with the frequency used. 2.4 GHz has smaller bandwidth than 5 GHz does and should be slower than 5 GHz. I just got the Orbi RBK852 myself. Claims are - 8 antennas, three bands - 2.4 GHz, 5 GHz, and a 5 GHz backbone. The 2.4 GHz band can handle up to 1200 Mbps while each 5 GHz band can handle up to 2400 Mbps. They don't say how they split up the antennas but it's not even. Eight doesn't divide by 3 very well. Also - note there is a 2.5 Gbps WAN port you hook up to your cable modem and it can handle that throughput to the internet if you have that kind of connection. I have to say - this system is awesome!
But getting back to your question, I've seen my MacBook Pro connect via 802.11ac between 527 Mbps and 10xx (in the 20's like 1025/1027?) Mbps range - it's one or the other. My other devices show similar type connections. All it means is they can connect, like connecting to a 1 Ghz LAN port, doesn't mean the network speed will be that fast. They will be limited in aggregate to what each band can handle.
On another note, the ASUS AI system also has the third 5 GHz band to communicate between APs as well. BUT - if you hardwire between them (Orbi doesn't have this option) you can use the third band as another 5 GHz SSID, which is pretty cool. I don't recall the data throughput though.
