It looks like they naively aligned the slant of the leftmost edge at the mid-height of the glyph, which is not the right way to optically align shapes, especially for a symbol with asymmetric curvature (the leading face of the loop leans substantially more than the trailing face). And then in addition to that, a too-wide arrangement when adjoining ("ul", "ll", etc) causes excess dead space around adjoined pairs.
You see this quite strongly in the "null" that appears on line 5 in their example screenshot. The two "l"s both appear to lean more than the "nu", and the "ul" has more internal space than the "nu", and the "ll" has even more than that.
For more info: https://github.com/tonsky/FiraCode/wiki/How-to-enable-stylis...
This seems bizarre to me, I do not understand the rationale behind it. Can someone enlighten me?
< Look at the tasteful colouring. The tasteful thickness of it. Oh my God.