After OTA update, Verizon iPhone 5 still using cell data on WiFi

Like the title of the post says, my Verizon iPhone 5 is still using cell data while I'm on WiFi, even after applying the OTA patch (according to the About section on my iPhone, the Carrier field is set to Verizon 13.1). It's not just a few kilobytes here and there; it's larger chunks that I can't pin down. I've been using DataWiz and Onavo to track my data usage. I've made sure to properly sync DataWiz's cell data usage metrics with Verizon's.

Here's an example of the issue:



My phone had been connected to WiFi since ~9:00 PM last night, and it wasn't disconnected from WiFi until ~7:40 AM today, so why is my phone still using cell data? And, yes, my phone was plugged into a power source the whole night or else my battery wouldn't be at 97% this morning.

Is anyone else experiencing this issue after applying the OTA patch?

P.S. Does anyone know why tinier chunks of data are used regularly while connected to WiFi? Do 3rd-party apps really need to use cell data to phone home? Or is it iOS services that are phoning home? If I'm connected to WiFi, shouldn't there be 0 KB of cell data being used? Maybe it's location services or something that has to use cell networks?