Did you have a positive tab credit as well? Because I received the same offer, but I have a $150 credit to upgrade my phone. And from what I can tell, they are only applying the $150 credit and not the $100 one.
Additional offer discount: -150.00
Tab Medium discount: -310.00
Due now: $0.00
Monthly Tab Charge $13.00
On top of that, my $40 plan is now considered a Small Tab plan, when a few weeks ago it counted as a Medium Tab plan.