Well I think I have figured this thing all out.
Just had my lunch break and during the break I was playing youtube videos, while eating. I was watching vids on the yiutube app for about 35-40 minutes, when I finished my "android OS" data usage went from 375mb to about 540mb after watching yt.
I gues yt data is somehow directly involved with the os data even if the two have their own seperate graphs in data usage settings. Now it would be good if someone can indeed confirm this on their fones just by watch8ng 10-15 minutes of yt vids