We definitely want to make sure your estimated data is giving you a good idea of what to expect. What sort of device do you have? Is this the estimated data you are seeing on My Verizon or through the My Verizon App? The estimated data should be very similar to the actual amount. The most common reason we would see a difference is if there is usage which hasn't reported yet.
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Keep in mind that any data incurred off the VZW network can be delayed. I've had this happen to me on several occasions. Thankfully I had enough data left at the end of my cycle that it didn't cause me to go over.