Hi,
As far as I know, the underlying SVG notation just has cubic splines, not circles. This would mean all things that look like circles aren't exact, though they may look round.
This is not true. See http://www.w3.org/TR/SVG/shapes.html#CircleElement
The SVG format has a circle element. SVG tries to be semantically rich as it wants people to animate things with ease - f.e. people want to easily animate the radius or center of the circle. Also, SVG is also meant to be used by hand-coders and programmers, and for them it is easier to use circles than cubic beziers.
As Tav explained, Inkscape uses arcs to represent circles and the Cairo library used to render the geometries uses cubic splines. But this is the decision of Inkscape and Cairo and not a limitation of the SVG format itself, which offers basic shapes, such as rects, circles and ellipses.
Andreas