Accidental clearance is wider when the note sits on ledger lines

An accidental placed to the left of a ledgered notehead must clear not only the notehead but the ledger line(s) passing through/near it — the accidental's glyph bounding box must not overlap the ledger-line stroke. Because the ledger line already occupies horizontal space at the notehead's exact y-level, engravers give the accidental slightly more left-clearance on ledgered notes than the ordinary same-staff clearance.

A sharp or natural sign whose vertical stroke visually merges with a ledger line running directly behind it, reading as a single thicker mark rather than two distinct symbols.

Gould, Behind Bars, Accidentals chapter, notes the general clearance rule; Ross, The Art of Music Engraving, explicitly calls out ledger-line notes as needing extra accidental clearance since the ledger stroke sits directly behind the accidental glyph's vertical stem strokes (sharp/natural/flat all have vertical elements that can visually fuse with a ledger line at tight spacing).

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.