It is true that the RG65 first started life as a 65cm boat in South America, born out of the free-saling G65.   But, to help builders, a tolerance was permitted of +/-0.5cm.   So that means that the hull could be 65.5cm long.

After a while it was decided that these boats needed a rubber bumper on the bow to stop boats form being damaged in a collision.   Rather that expect everyone to change their existing hulls it was agreed that the 0.5cm rubber bumper could be added to the front of the existing hull.   Once again, to help builders, it was agreed that a tolerance was allowed for the bumper of +/-0.1cm, therefore allowing a bumper to be up to 0.6cm long.

Add maximum hull and bumper and you get 65.5 + 0.6 = 66.1cm.  

So, nothing has changed in the length measurement, only the way it is defined.