So this is a question verizon hasn’t answered yet. I noticed on my bill that all but one phone is 20 dollars a month for data. The one that is not is 40 dollars a month. Besides being newer, the only difference I can think of for this phone is that it isn’t bought off yet. Are phones that are on the monthly plan for paying off, charged more for data? That is all I can think of. We had a phone from 2015 that was 40 for data a month, then after two years it became 20. That is the only thing that I can think of, of why I am being charged 20 more for data.
Not sure if this applies to your plan, but when I had my phone on my parent’s AT&T plan, it was the “main” one (as I was the one who set up the plan originally). When I called the company and asked them to explain, they simply stated that it was due to my phone being the main phone number for the account. I am sure there was little to no truth, but at the time I accepted the answer. I hope this can help you get a more direct answer to your question as well!