With certain Ridley frames, there appear to be two different size progressions in a given model: even # and odd # sizes. So looking at a 56 vs 57 would result in vary different geometry, where a 56 vs 58 is similar geometry but scaled up.

I haven't owned a Ridley, but have eyeballed them, and if you go to Ridley's page you can see which models do this and which don't.