I asked the same question, apparently its because Gigago is treated exactly like any other "open access" ISP.It looks like I will be receiving the same equipment for the Gigago connection.
I find it quite strange that Gigago can't take over administration of the current Calix 813G that is in my house. It would be far cheaper this way instead of sending a vehicle to collect the 813 & then sending another vehicle to deliver the 804 & 844.
I think this hasn't been thought through by management.
On the other hand you would still need an 844E as the 804 does not work with the 813, it needs the 5Ghz radio to use as backhaul.