Lightsail Data Count in Bills, Overage Confusion

Most recent bill is the first time I have incurred overage, I am confused about how data is listed in bills and Cost Explorer for Lightsail

The instance is a $5/month 2TB instance.

October shows, that the nstance used 3435.785 GB Total Data Xfer Out, and was not charged any overages.

November shows that the instance used precisely 3000 GB Total Data Xfer out, plus 390.457 of Overage for $35.14 overage charges.

So I'm not understanding

1. gbPerMonthAllocated for the instance is as expected 2048for the $5 instance, but this doesn't match with what is in the bills/cost explorer, at all.
2. October's bill shows more usage, than November, but I was not billed overage in October (not that I want to complain!), but was in November.
3. Given that Cost Explorer and the bills don't seem to match with actual quota usage, what is the best way to actually see how much of that 2TB quota an instance has used at any given time so that it can be tracked and determine if an overage incurred in a month was due to some one-off event, or if it is an ongoing condition which needs attention.