Why do scroll bars have two different behavior?

In Debian 10, but since older versions (always using Gnome), the scrollbar usually follows the mouse (the scrollbar knob is always below the cursor), but sometimes it scrolls the page contents much slower (the knob lags behind the cursor - e.g. the cursor have "walked" 500 pixels, but the knob only "walked" 50 pixels). Not a Gnome image, but you got the idea of what I'm calling "scrollbar knob": Most of the time I get the first behavior. Rarely, and unpredictably, I get the second. Actually I'm more used to see it happening on vertical scrollbars, but I guess it may happen equally on horizontal. And it usually happens when the content is much larger than the window, so that the knob is actually much smaller than on the image. After five years using Debian+Gnome, I still can't figure what causes the difference. Can anybody explain it? I didn't find anything on Google (maybe I don't know the right words to search?).