Join Date: May 2003
Location: Orlando, FL
Bikes: Scott CR-1, Serotta Legend, Serotta CR, Co-Motion Speedster tandem, Masi Nuevo Strada fixie
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I'm betting it's a mistake
I'm betting it's a mistake on the website.
The Ritchey Comp 30 does not have the conflicting data that is apparently on the Pro 30. My guess is that some webmaster cut and pasted the description from another stem and forgot to edit out the part which gave the choice of angles.
As far as explaining rise, the chart that dfcas links to is pretty self explanatory. Keep in mind that the stem angle in degrees is not measured from horizontal. They are measured from the normal (right angle) to the steerer tube which is already angled up around 16 to 18 degrees, depending on the head tube of the frame in question. So even a stem with 0 degees of rise (90 degee) will still bring the bars up. This is covered (somewhat cryptically) in the note on the Habanero stem chart.
Enjoy your new tandem.