Confusing internet usage reports

In September, near the end of the month, I received a notice that I was nearing my 1.2 TB data limit on my account. I began monitoring the usage and on September 30th at 10pm I had used 95% of my maximum.  This was the first time that I had received these warnings.  Now when I look at the usage graph on the app, it shows that I used almost all of my allotment in August (about 99%) with no warnings and when I checked during the month I was no where near on target for that type of usage. It also shows that I used almost twice the max in September.  With my speed and the report at 10 pm on the 30th, this is impossible. If I check the data usage in my account on the web, the graph is much more accurate, but the data table below the graph shows something similar to the phone app, but reports no overages.  There is something that is out of whack with this and it seems that it is impossible to accurately monitor this.  Why is this the case?