Hardware issues aside, you may want to consider installing some type of bandwidth monitoring software. At minimum, you'll have a secondary log of data usage to compare against Verizon's logs. In any case, you need to determine what is specifically using your data bandwidth if you're going to fix the issue.
NetWorx is a popular free app. Besides daily, weekly, and monthly data stats, it also displays bandwidth usage by applications. Though a little more involved, you can optionally set data monitoring from your router(MiFi), instead of your computer.
NetWorx does allow one to set notifications based on data usage, but doesn't provide a way to actually control or set bandwidth limits. There are a few free apps that allow basic bandwidth limiting. But, a little more involved in setting it up.