Ondrej,
SwitcherPro's power stage gain and phase numbers include the effect of slope compensation. The datasheet equations are simplified, and the datasheet even mentions that the datasheet compensation procedure ignores slope compensation (see Compensation section on p.34 of the datasheet).
In reality the DC power stage gain (including slope compensation) will take the form of GpsDC = gmps / (1/Rout + go), where go is a term introduced by the slope compensation. This extra term is causing the gain to be lower than you expect.
Kris