Nokia Lumia 920 Data Usage - Strange usage calculations going on

I wanted to see if my data usage would go up while on WiFi all day at work. I laid my phone down on the wireless charging pad (WiFi stays connected ONLY if charging) at 7:41MST this and I haven't moved it since then. Here are my results

7:41 am 82.34 of 3,072

10:21 am 88.36 of 3,072

1:59 pm 264.15 of 3,072

My data usage jumped by 175.79 MB while on WiFi between 10:21 - 1:59... has anyone else experienced this or can you please test as well? Do all the apps go through WiFi when N920 is connected to WiFi or do some apps still go through AT&T.. i.e. Skype