> The offset of each seems to have no rhyme or reason
As others point out, those are lowercase numbers, adjusted to look better alongside lowercase text. The rhyme and reason is in how the digits fit within x-height of the font. If you look here: http://en.wikipedia.org/wiki/File:Mediaevalziffern.svg, notice how the dominant "circular" part of each digit fits within the x-height. The digits 3, 5, 7 and 9 don't have a dominant circular part, so their lowest arc is considered a descender, similar to how a 'g' or a 'j' is rendered.
As others point out, those are lowercase numbers, adjusted to look better alongside lowercase text. The rhyme and reason is in how the digits fit within x-height of the font. If you look here: http://en.wikipedia.org/wiki/File:Mediaevalziffern.svg, notice how the dominant "circular" part of each digit fits within the x-height. The digits 3, 5, 7 and 9 don't have a dominant circular part, so their lowest arc is considered a descender, similar to how a 'g' or a 'j' is rendered.