It can go the other way, too. It is mathematically impossible to make a font that has the exact same vertical metrics/line-spacing everywhere, so everyone sets

It can go the other way, too. It is mathematically impossible to make a font that has the exact same vertical metrics/line-spacing everywhere, so everyone sets the three value sets provided by the OpenType spec to the same values for web fonts. And then you try to render a font with tall glyphs (i.e. Arabic) on Android and find that it actually looks at the font-wide bounding box in the head table to determine metrics. You can't change them because then Android clips everything legitimately outside that box.