Jony Ive’s new “icon grid” is a guide meant to ensure that different apps’ icons look harmonious on the home screen. That’s a lofty goal. The issue of whether a grid can really accomplish that is complex; most designers think that non-block-based designs (so, not paragraphs of text, not photos, not headings) require a lot of “optical adjustment”. This is fancy talk for “tweak it so it looks right.”
Astute. It’s the same reason why monospaced fonts don’t look “right” for body text; different letters require varying amounts of space to appear visually similar.