CE investigated this topic on the Kinhank G1 box some month ago.
Indeed, it’s strange that they assemble HW PHY for gbit ethernet, but limit it by SW to 100M. We tried to get gbit running by changed dtb, but result was: No RX packets.
So we think they intended to have gbit, but have a buggy pcb layout. Therefore they had to limit the speed to 100M, to get sales running.
You may ask Kinhank directly about the reason, and if they intend to release a new box rev. with gbit. There is nothing CE can do here…
[EDIT] Kinhank never specified the Ethernet speed of the G1.
But maybe they already fixed the issue and released new rev. of the box, as some users could get Gbit running (on Ugoos X4 DTB)