I expect the reason is that the Wi-Fi disc is a far superior Wi-Fi client than your Xbox 360 so it is capable of much higher connection speed.
I recall the Xbox 360 has a 100Mbps Ethernet port (which will max out at around 95Mbps actual throughput).
Now compare that with the Wi-Fi connection on the disc which is AC2600. The 5GHz backhaul channel is Max 1733Mbps. In practice the Wi-Fi backbone will give you up to 200Mbps in a typical setup.
So if you connect the Xbox 360 by Ethernet to this disc, the bottleneck is your 100Mbps Ethernet port so yes it will be similar performance to connecting Ethernet to the Hub but the latency will be a lot higher (ping time) as Wi-Fi latency is higher than Ethernet, especially on 2.4GHz.
Now consider the Xbox Wi-Fi adapter is either 802.11g or 802.11n depending on how old it is. If its using 2.4GHz that's 54Mbps 11g or 144Mbps 11n. As Wi-Fi is a shared access technology, that is shared so in practice you won't get those max rates. 2Mbps seems very low for Wi-Fi but it may be connecting direct to the Hub with a weak signal rather than the disc.
If Ethernet is available in that location, I would recommend connecting your Xbox 360 that way based your results.