The reason this happens is because the driver switches between USB 2 and USB 3 modes as needed.
These devices are backward compatible with older networks that have speeds below 480 mbps, such as 802.11n 150 and 300 mbps networks. The default mode of these USB 3.0 devices according to diagnostics is USB 2.0. When you are not connected to any network, this is the mode that will be displayed. When you connect to an 802.11n network, or a 1T1R 802.11ac network, the adapter will stay in USB 2.0 mode because the max speed of 480 for USB 2.0 is more than your network connection rate.
But connect to an 802.11ac network above 480, such as an 867 mbps 802.11ac 5 GHz network, and then check diagnostics again. Now it shows USB 3.0 as the mode. The chipset uses smart switching to go into either mode depending on what type of network to which you are connected.
New this week on Rokland.com is Alfa APA-M25, a 10 dbi dual band 2.4/5 GHz directional Wi-Fi antenna with RP-SMA connector.
This is compatible with many of our Alfa Wi-Fi USB adapters, including the new 802.11ac standard AWUS036AC model from Alfa.
We have the antenna for sale by itself, and also bundled with a magnetic mount antenna docking base + 3ft extension cable.