I figure out what caused this.
It is something to do with font settings with Google Chrome.
I found that my "Minimum font size" is not at its minimum
(It was 12px before I changed it, now 6px.)
(not sure why, I am pretty sure that I didn't touch this setting before.)
But even though I changed minimum font size to its minimum, the Ads still looks not correct (compare to other's pictures).
https://i.imgur.com/gAQtRo9.png

But it is a good thing that scroll bar at the bottom is now solved.