I, too, have been getting false data alerts for months now. Yesterday, I received two: the 75% usage and 90% usage alerts. Checked my account online and am not even close. In fact, my billing cycle is only a few days old and my actual usage is 0%!
I reported this problem several months ago and, like you, I was (correctly) assured that the problem was with the alerts, not my usage. But I have little confidence, after this length of time, that solving the false alert problem is high priority for Verizon Wireless. And it does defeat the purpose of the alerts if we have to take the time (and it does take significant time) to access our accounts to check our actual usage because we can't trust the alerts!