The font for this test case has a single glyph (‘H’) which displays examples of a boundary IUP issue which was treated differently in existing implementations. The top of the top serifs and bottom of the bottom serifs all have multiple points at the same Y location in the default. If you examine the bottom left serif, you will see points 149 through 157 all share the same Y coordinate of 0. When moving from the default 0.0 to –1.0 in the Hooves axis, point 153 moves by (0,50). This now leaves 6 untouched points that need to be interpolated. The handling of these intermediate points (150–152 and 154–156) was different between Apple’s, FreeType’s and Monotype’s implementations.