Why do Three cylinder engines have unconventional cc's ?

[simonm]

All the triples seems to be 675cc rather than 600cc.  Why is there such a marked difference between a cylinder of 200cc and one of 225cc ?  I would have thought they could tune the engine, exhaust/weight to make any configuration work.  Obviously I'm wrong as Triumph and Yamaha have gone the 225cc route.


Does anyone know specifically what it is that makes the extra 25cc so important ?


cheers
Simon