I think you have the wrong idea about percentages.
If something has a droprate of 10%, that does not guarantee that you will get one after killing that boss ten times. Every time, you will have 10% chance of getting it. It's not like you get 1 every 10 kills -.. Same goes with Mog Scrolls on BGs. If it has 0.1% chance drop rate, that doesn't mean, after doing it 1000 times, you will get 1 scroll 100%.
Every BG victory, you will have a set of chance of getting the scroll and every BG victory is unique from other victories.
One day, you get one after doing just one BG because you are lucky. Some days, you get one after 2000 BGs because you just are out of luck.
I myself don't do BGs for Scrolls, but I got two from this week's ICC 10, which is extremely lucky for me because I normally get one per month.