The leading edge is not a circle. Its radius is as specified only at point zero on the X. The curve has a constantly changing radius away from 0, which a circle or arc does not. It's a pity MoI does not have a curvature radius display then it would be more obvious what I am talking about. An ellipse is a better approximation than a circle. Indeed it is suggested that a tilted ellipse is the way to get the first section of the aerofoil. You do not want tangent connections either. They must be G2 curvature continous at the very least.