[Fontconfig] Why is the default weight FC_WEIGHT_MEDIUM?

[John Flatness]

I was recently trying to run down a quirk in font selection, and 
stumbled upon Fontconfig's pattern defaults. In particular, the one that 
was causing my issue and surprise was the default for weight, which is 
defined as FC_WEIGHT_MEDIUM, or 100.

This default seems to have been the same from the initial commit, and 
that time there were only defined constants for light (then defined as 
zero), medium, demi-bold, bold, and black weights.

By now, should the default weight not be FC_WEIGHT_NORMAL or 
FC_WEIGHT_REGULAR, both of which are defined as 80? The specific context 
I noticed this in was a font family that ships both regular and medium 
weights, and without additional specification, fontconfig selects the 
surprisingly-heavy medium weight.

I'm aware I can provide my own configuration to handle my specific case, 
but I'm surprised that the default here is actually heavier than the 
"regular" and "normal" weight. I imagine there's some consideration here 
that I'm simply not aware of, but I can't think of what that might be.

--
John Flatness

(P.S.: I tried to send this message earlier, and it seemed to get eaten 
somewhere along the line. Apologies if this is a duplicate for anyone.)