Over Charged

I have a prepaid account with a second phone on my account. Both phones are under the $45/month plan (multi-line discount -$5) and I also have auto-pay (-$5 @ 2 = -$10). My total bill Is $45 × 2 = $90 - $5 - $10 = $75 × $6.19 (0.0825 tax) = $81.19. An $81.19 deduction was attempted from my account - twice - on the day my payment was due. I received a text message saying the attempts failed and they would try again in 24 hours. I contacted my bank because the payment should've gone through. My bank told me to try again. So I tried putting money onto my account manually, as I've done before with auto-pay, but my card was still declined. Knowing my phones would be shut off the following day, I took my cash and went to purchase 2 - $45 prepaid cards from the store so my service would not be disconnected while I straightened out my bank account. I loaded the 2 - $45 cards, giving me a balance of $90 on my AT&T account. The $90 was loaded to same place I've added money before from my card and AT&T used that balance instead if deducting my account with the auto-pay. Immediately my AT&T account balance was paid. Today, I checked my AT&T balance because I tried paying for something on Google Play ($1.99) with my AT&T balance (which should've been $8.81 ($90 - $81.19). My balance is $5. I'm still enrolled in auto-pay, but my payment history says I was charged $85 this month instead of $81.19. It's only $3.81, not a big issue. I'm just wondering why? I'll ask a different thread why my $1.99 charge was declined with a $5 balance. Thanks!