When I'm using the web version of the guide in full screen mode and scroll pages up or down, it over scrolls by about 3 rows. My monitor is 1920 x 1080 scaled 100%.
I'm not sure I follow you - there are no page up/down buttons. What exactly are you doing? If you're clicking the small up/down arrows at the top/bottom of the scrollbars, these are not page up/down buttons. Their behaviour is defined by the browser, and typically only go a small portion of a page at a time.
The web UI is intended for being driven by a mouse. To scroll you'd typically either grab the scrollbar and drag it up/down the screen until wherever you want. Alternatively you can use the mouse wheel.
You can probably press the PageUp/PageDown keys on your keyboard to jump a page at a time.
I can see a slight issue that might be related. In the first capture channel 9 NBC is obscured by the bottom scroll bar. After paging channel 9 is mostly by the top scroll bar. Also page 1 always seems to have one less row then the next pages.