Monthly charge for service not consistent

Since T-Mobile would not absorb the rate hike for including "free" Netflix, it now has ads unless you want to pay $8.50 extra per month. I chose to pay because I hate ads! Unfortunately, my monthly bill keeps changing month to month, a dollar+ or -dollar. T-mobile tells me it's because of the taxes on the $8.50 I pay for no-ad Netflix. They have no explanation as to why Netflix taxes change from month to month.  There are taxes attached to every utility we pay but never see them changing monthly. Does anyone have an answer as to why this is happening?