Tethering PS4 through VPN...and Tmobile still knows!?

Hello Everyone.

I am using a VPN and the Wifi Tether Router paid app from the market. I am curious as to how T-mobile still knows I am tethering, as I just got a "You used 80% of your High speed data" text from them (Have unlimited data plan)

Any ideas on how to completely hide? SSL? Proxy? Only seem to have issues with the Playstation and not the PC....

Any help appreciated...