I was looking at DataSense data in my phone and suddenly saw this strange behavior:
As you can see the Internet Sharing used 268MB of cellular data and 279MB WiFi.
What it means? If I'm sharing internet how can WiFi be consumed?
You are tethering over WiFi so the input is coming from data and the output is going over the WiFi network.
268MB of cellular data and 279MB WiFi will always stay pretty close together.
The question about why they differ at all is a lot more complex and is probably best answered by the developers of the app.