Why is the sales tax NEVER the same??

The sales tax (located in Texas) on my monlthy subscriptions, which include Fitness+ ($9.99), Apple Music ($9.99) and starting today will include Apple TV+ ($4.99, but as I have yet to be charged, I don’t know the tax on that one) is 8.21%. So, the total amount for Fitness+ and Apple Music are $10.81 each and that has never changed. I also pay $0.99 monthly for extra iCloud storage, but that’s not actually a subscription, and there’s no tax included on that one…

So, I recently made 2 in app purchases, both for the same app and made within a week of each other. One of the charges was for $5.98 with a sales tax of 8.19%, equaling $6.47. The other charge was for $11.98 with a sales tax of 8.18%, equaling $12.96.

So, my question is, how is it possible that the sales tax for the in app purchases be different? And not only that, why is the sales tax higher on the cheaper purchase than on the more expensive purchase? Can someone make sense of this? It’s just confusing when the taxes are always different…

I’m assuming the reason the sales tax on my subscriptions never changes is because they are directly through Apple and not a third party, which makes sense.