Wrong Bill Calculation in T-mobile One Plan

I have been using Tmobile for last 8 years. This is the first instance of Tmobile doing wrong calculation for my bill. I went from 9 lines to 8 lines but my Account Bill remained same. The line that we removed is not a free line and it stayed with us for 6 years. I spoke with customer care multiple times but they are not helping me and saying everything is right.

I have 8 lines on Tmobile one Plan so Account Bill with autopay should be 70+50+6x20= $240
Insider discount 240 x 0.2= $48
Account Bill = 240-48 = $192 for 8 lines -- This should be my plan cost but it is $216

I want to know if you think my calculation is wrong. Please let me know how can I get this sorted out.