rounding edges

 From:  TOM (SIRTOM)
7844.19 In reply to 7844.2 
Dealing with filleting I realized that it is not possible to round with a perfect circle - for example if
you have a distance of 2,00 it is not possible to fillet with radius 1,00 exactly but a bit smaller than
the radius needed for a perfect circle.

Is there a specific reason behind it ?

EDITED: 13 Feb 2016 by SIRTOM