CSSL position:fixed scrolls a few pixels, how to win?

Question

css

You need to open from a mobile phone https://goo.gl/Gkc4hw (or narrow the window to 500 pixels)
The top red block is specified as position: fixed, but if you scroll down the scroll, it shifts a little. Why?
We are talking about this red block on top

There is no problem on this page - https://goo.gl/V7Ju32