Oh I know how it's supposed to work. 

No I'm not using all of it each month.

This month will be perfect for me to check on how they're playing.  I was up north for 4 weeks, used 2 GB of my 16 and had only 1 "bonus" GB from the previous month - not rollover - they gave it to me.

So, this week I rolled over 4.69GB which gives me 20.69.  So far, I've used 2.12 and do have another week or so of time that I will be up north.  I will lose the carry-over and, if it works the way it did the last time I had original data left untouched - I will start out Dec 13th with just 16GB. 

I pay real close attention to my usage since mobile data has been my only source of Internet since 2008.  This is not new to me at all and I pay real close attention to anything that I'm paying for and will complain if I'm paying for something I'm not getting.