Butted 2.0/1.8/2.0 spokes are stronger than straight-gauge 1.8mm spokes. And as Retro Grouch points out, they're stronger where it matters - at the elbow and the nipple.
What's interesting here is that these butted spokes actually make for a stronger wheel than straight-gauge 2.0mm spokes - because the center section flexes more, which means lower max forces at the nipple and elbow, and on the flange and the rim.
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"c" is not a unit that measures tire width