Data for say TODAY... is not polled, until TOMORROW.
So if you received a notification ON the 31st, you were likely AT or OVER the 100% at some point on the 30th. (could have been, say early morning 30th).
So you were at about 1+ days of usage, over the limit.
There are many people who beleive the counters are wrong. Weither that is right or not? I cant say.
I have found, that they are usually pretty accurate myself. (i tend to monitor my usage daily.. how much downloading, game playing, netflix, etc.. and take averages for what those should consume.. and they are pretty much within the right ballpark.. prob +/- 200mb of what i guessed.). But that may not be the case for all.
Often big spikes in usage, can be from stuff internal.. not always what we are aware of, etc.
(Big game downloads/updates which can be 500=20g.... windows or other system updates. Cloud services are a BIG one if they are downloading/syncing).
Just the other day, i logged in and saw like 4gb worth of upload for one day.. which is VERY odd for me.. Oh yeah.. that was the day i forgot, and left a torrent running for like 5 hours seeding :(