Purchasing Extra Mobile Hotspot

My T-Mobile plan comes with 15GB Hotspot but I want to purchase more for my phone as I need to use more every month. I went ahead and purchased 60GB hotspot for $25 as an add-on feature for my Android phone. When my hotspot usage was at 60GB, I received a text from T-Mobile telling me my hotspot speed will slow down after 60GB was used up. I called T-Mobile and asked why wasn't my account has a total of 75GB instead of 60 (15GB with my plan plus 60GB that I bought extra.) I was told that if I haven't used any of my 15GB that came with my plan and I decided to purchase extra 60GB hotspots, I would lose that 15GB that came with my plan and only have 60GB only. To me, it's crazy, why would I lose the 15GB if I am going to buy some extra GB hotspot? I should still have the 15GB that came with my plan PLUS the 60GB which I paid extra for, which means I should have a total of 75GB monthly. I was told that's not how T-Mobile works. So instead of adding the 60GB hotspot add-on onto my account, I would have to use up my 15GB first, then buy the extra hotspot EVERY TIME.  To me, that's crazy logic.