Increased bill by $51 after upgrading with +$1 promotion

I upgraded from preferred $81 to ultimate $82 with +$1 promotion on 8/3. $1 difference would not affect major increase in my next bill. But OMG the next bill due by 9/14 (8/23-9/24) is $133 ($82 plus the partial month service $51). I donot understand where the partial month service of $51 came from. I think this may be a bug in calculation when I upgraded a service in the website. Do you guys have any idea that I missed or similar experiences? Thanks.