@Katsunami: Because they use a number of hard-coded stuff to tweak (at the very least) Caecilia's rendering. The most obvious one being Caecilia Condensed.
Try rendering the Caecilia Condensed font files with vanilla Freetype: it's not really condensed at all

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That doesn't really answer why they do it this way, but it at least somewhat explains the situation

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At least we got a pretty good full serif font in the form of Palatino with the PW... And Caecilia itself is also a pretty damn good slab serif. I'm mostly happy with the fonts available, with the glaring exception of Helvetica, which I simply can't stand (so I was pretty glad with the switch to Futura in most of the UI on the PW).
What I dearly miss is decent control over the margins, line-spacing, font sizes, and hyphenation, possibly contrast & hinting tweaks as a bonus.
Being able to tweak the renderer and/or font files to be able to control those felt a bit overkill

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(Wouldn't have stopped me though, my desktop ft+fc setup is pretty particular, and it's a topic I feel strongly about

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