The original calculation yielded 4.9 knots, so there is a net loss.
In lighter wind he has more wetted surface, and although his ultimate hull
speed may be
higher, he has to carry full sail in well over 20 knots to get there. The 20
knot
prediction leaves him at 1.2 sqrt(LWL), or 6 knots, instead of 6.3.

As neal says, math tells the truth.

RB