why the "a" italic in myriad pro is different than the in all other regular-bold-etc ?
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A "single storey" lowercase 'a' glyph is very common in italic styles of typefaces. The glyphs are designed like that to imply a more cursive appearance. Other italic glyphs have their own unique design cues. It would not be the preference for a typeface to simply shear upright letters into a slanted appearance and pass that off as an italic style.
More info: https://en.wikipedia.org/wiki/A#Typographic_variants