Are you looking at the detailed billing? Open the PDF and see if it happens there. If it does then you should be refunded the difference. If it doesn't then it's just a visual error.
If you need more help getting some clarification on the bill, I'd suggest you contact our T-Force team via the Facebook, Twitter, or Google+ links below. They are available 24/7 and have the ability to access your account to help shine some light on the charges you're seeing.
Many taxes are applied in this manner. It is not uncommon, and this issue has been discussed in the past. Furthermore, Taxes can be imposed at the federal, state, local, special district, and other levels. T-Mobile will never give you a straight answer about tax assessments (believe me, I’ve tried). If you really want to know exactly how all the taxes are being assessed, you will have to do a fair amount of your own research.
I suggest calculating what the overall tax rate is for you bill, then comparing it to what it say in this link:
The above link will list a tax rate for your state. It will not be exact, as local taxes usually apply, but it should be in the ballpark.