Well 'mod' means modified, and 'stock' esentially means standard. So the stocks haven't been modified for trials, thev'e only been adapted (geo, ect...) Mods on the other hand have actually been modified for trials, and the reason behind the rims (as muel said) is that if you were going to whack a massive 'trials specific trye' on a rear 20" rim, your rear wheel and front wheel would now be different sizes - meaning it would feel awkward to ride! So by shrinking the rim by an inch (down to 19") you can keep the 'trials specific tyre' without messing up the geo and ride height of the bike. Make more sense now?