This is the last I repeat. All you're saying is that you get rollover each month that you don't use all of your allotment. Meaning that you're not using the rollover.
If you use any of your rollover, the next month, you don't get any rollover because you would need to go through your allotment before you can use rollover.
If they used the rollover first, then you could use the rollover each month (provided you don't go over your rollover and allotment). For example:
1st month with rollover (using your 20 gb a month allotment) with you having the 31 gbs to use:
20 allotment
11 rollover
31 total
To use any of the rollover, you have to go through your 20gb allotment.
Even if you only use 20.1 gbs, you get no rollover for the next month.
If they used the rollover first, you could use rollover each month:
11 rollover
20 allotment
31 gbs total
Here, you could use 20.1 gbs and still have almost 11 gbs to use the next month.
This is lame and ATT knows it. Actually, it's pretty clever on their part but it's still lame.