Generated obliques have no hinting applied to them
Below is an animation of three lines of Cantarell, at 13px, 15px, and 17px. As you can see, at these sizes, the cap height and x-height of the text is up to one pixel shorter in the italic version of the font, which makes text look mismatched when normal and italic words are mixed: . This is not an issue at larger sizes, which is why I blame hinting.
Here is the same image, zoomed 4× (click to see at full size).
This text was rendered in Chromium on Arch Linux, using cantarell-fonts-1:0.101-1. I believe this is a regression from the 0.0.25 release (at least, I never noticed it before upgrading). My hinting setting is ‘slight’ and my antialiasing ‘grayscale’.