Simplified version:
AT&T typically pays $350-$400 for you to get a new contract.
The money that AT&T was setting aside to pay off your gets added as a discount.
If AT&T helps to pay your phone, you stop getting that discount, as that money now goes back to paying it off.
Before, AT&T would keep the money, regardless of whether you paid off the phone or not, you'd just pay a consistent flat rate. Now you get that money back as saving when the phone is no longer a concern.
$145 is pretty competitive for 3 phones, but check www.myattrpc.com to see if you don't have a better alternative.
-Alex