Im p sure data usage is dictated by various other processes too.
You are Implying the following:
1. You only had minecraft open.
2. The data was not being used elsewhere.
3. That the data shown is factual.
I looked at my data usage in Windows settings, and the vast majority of the data was used by javaw. I did only have Mc open as far as I can tell, but I suppose there could be an inaccurate measurement of which apps are using data.