
Joana |

Isn't it referring to the vertical drop, while the grid is depicting horizontal space? Like the edge of a cliff, in a top down map, has no horizontal distance between it and the land at the bottom, but it could be a ten-foot drop or a hundred-foot drop without "taking up" any squares of the map grid. The vertical dimension is given in a note so you know how much falling damage someone takes if they fall off.
Or am I misunderstanding what you're asking?