The above is based on the assumption that
Static Thrust ~ D^3 * RPM ^2 * W * CL
where D = Diameter
W = prop cord width
CL = prop airfoil coefficient of lift

The Thrust HP calculator assumes slightly differently:
Static Thrust ~ D^4 * RPM ^2 * CL
I think they take "W" out of the equation to make it simpler for us users. They assume that W will increase proportionally with D, which is true in some cases. But when you use Thrust HP to calculate a wide-blade prop, you have to adjust the cord width accordingly.