Do you think maybe T-Mo has an additional threshold - the Nag threshold?
So we know that you get 3.0GB at full speed, then we get throttled to 2G speeds. Then you added 10 more GB (13.0GB total). So maybe their system works something like this?
* 0 to your plan's 4G limit: full speed
* Next 10GB: 2G speed
* After that: Nag screen for non-mobile device
I would be curious what happens if you use one of the apps that redirects your hotspot web usage through their app to make it look like "phone data". Under this scenario, you'd have 2G speed, but no nag screen (if the data is sufficiently cloaked).