I've pondered the same thing recently. All of my short pull levers have a radius of 27-28mm. My Shimano long pull levers, at least the ST-EF500s that are switchable between short and long pull, have a long pull radius of about 40mm and a short pull radius of about 28mm. I also have a set of the Avid FR-5s (supposed to be long pull) and they're like yours: in the middle at about 32mm. I also have a generic set of long pull levers and they're also about 32 or 34mm as I recall. I haven't found any as short as 21mm.

I don't know if the Avids are shorter than typical for long pull to provide more mechanical advantage...or if the Shimanos are longer than typical for long pull to reduce mechanical advantage (such as for inexperienced riders on low cost bikes).

Anecdotes:

• I'm currently using 28mm levers with Tektro Mini-V brakes on my Trek 750 and I love this combination.
• My daughter's bike came with Tektro Mini-V brakes with Tektro long-pull levers (I think they have a radius of close to the Shimanos...near 40mm); I swapped regular (longer) V-brake arms onto it to get more mechanical advantage and the brakes are definitely stronger.

Good thread! Totally nerdy and something I've been curious about myself lately.