My current plan is 2GB with rollover. Last month I used 2.5GB, and somehow this month has 2GB included + 2GB rollover = 4 GB available which by itself is also weird. Maybe most of the 2.5 GB I used last month was rollover from the month before that or something.
In total I have used 4.6 GB of mobile data so far this month, which is more than the 4 GB (2+2) I have available for this month and it’s still working.