FontBBox, Ascent, Descent

From Pdf 1.7:

"FontBBox rectangle (Required, except for Type 3 fonts) A rectangle (see Section 3.8.4, Rectan-
gles), expressed in the glyph coordinate system, specifying the font bounding
box. This is the smallest rectangle enclosing the shape that would result if all
of the glyphs of the font were placed with their origins coincident and then
filled.

Ascent number (Required, except for Type 3 fonts) The maximum height above the baseline
reached by glyphs in this font, excluding the height of glyphs for accented
characters.

Descent number (Required, except for Type 3 fonts) The maximum depth below the baseline
reached by glyphs in this font. The value is a negative number."

Ok, so it sounds like the fontbbox y coordinate should be equal to the descent, and (ascent - descent) should be equal to fontbbox height. However in practice this does not seem to be the case at all.

The type1 font that I'm looking at right now looks like fontbbox y is correct, and ascent is correct. So I can find the bounding box height by (ascent - fontbbox y).

Ascent: 702 Descent: -180 FontBBox: -167 -229 1000 930

So, which *should* take priority? descent or fontbbox y?