This is just our old friend `pango_context_set_round_glyph_positions`. Try it with `TRUE` instead of `FALSE` in `scintilla/gtk/PlatGTK.cxx` and each `U` will look the same. While that looks more consistent for the example, it doesn't always look better and the layout is less 'correct' with positioning discrepancies more likely.
The pattern of 5-10 instances being identical then moving to a different pattern could be something like aliasing to fixed-point positions (like an eighth of a pixel) instead of using true floating-point. I've considered doing this in Scintilla to save space.
Scintilla does not place each character individually - instead it asks Cairo and Pango to draw whole segments of text. There is also no possibility of the text being expanded/contracted to fit within the width allocated by Scintilla since the width (or right ordinate) are not passed to Cairo/Pango here.
@elextr: I can't actually see any significant difference
There are layers of image resizing and smoothing getting in the way. Save the image into a file then use a bitmap viewer (like Image Viewer with the 'Smooth images' options turned off in Preferences) with integer magnification to look at the image closely.