Leading is determined by the font or fond. A character cell is a matrix of pixels that determines the maximum size of a character. If there is a cell matrix of 10 X 10 and there are two rows of unused pixels at both the top and bottom of the cells, then there will be four rows of blank pixels between the top and bottom of the characters since the cells are stacked vertically on the page.
Leading would add blank rows in between the character cells. Leading is set to zero in the Macintosh. In your terms, the leading is determined from baseline to baseline because the cell size is not modified. The LaserWriter driver does not convert to output to 72.27 points.