brightness adjusts while scrolling?

[stikfigure16]

I've noticed the brightness adjusting slightly when on my home screen and while scrolling through comments on a Facebook post. It only happens when I hold my prime vertically though. On the home screen if I touch the screen and drag my finger slightly to the left the screen will slightly dim and when I let go the brightness goes back up. On Facebook when I scroll down the comments on a post the brightness dims slightly and when I go back up the brightness goes up.

It is not super noticeable and this is isn't a major issue, but I was just wondering if anyone else has noticed this. Thanks.