I am having the same issue, My husband only uses his data when he is home and on the wifi, we just got our phones and are on our first month. We got an alert stating that we are over the 90% usage limit and when I looked up our usage, it is racking up some serious data usage under his phone, which is when he is here and connected to the wifi. We were told at the store when we bought our ophones that as long as we were on wifi data, our plan wouldn't be charged for data usage. My husband has been diligent on always making sure that he's connected to it when he's using his phone, so not sure what the problem is, but it needs to be figured out. Is there a setting we are missing somewhere that needs to be turned off? Were we informed incorrectly? OR is it just a glitch with Verizon? 
It seems to be just like the issue that many are having with the iphone 5's.