This is just how sliders look with the theme I am using on my Linux. Those are ordinary Qt sliders, and by default they will look like ordinary Windows sliders on Windows.
I actually implemented those sliders (quite some time ago) and we were aware of the issues with its usability back then. I had hoped having this increase from left to right would be reasonable intuitive (at least for people used to a left to right writing direction). I had originally imagined those sliders to be somewhat formed like a wedge, but my Qt custom widget skills did not allow for that. Unfortunately nobody ever came by improving this implementation. Maybe after the Qt5 port is done this can be easier