Apsidal precession

There is the formula:

$$ Deltatheta=-2frac{partial}{partial p_{theta}}left[int_{r_{min}}^{r_{max}}sqrt{2muleft(E-V_{eff}right)}drright] $$

That indicates the change in angle for the perihelion (in the case of the Earth and the Sun) in one radial period. We can prove it is equal to $2(theta(r_{max}) – theta(r_{min}))$. Shouldn’t it be equal to $2pi$ in the Kepler case? It contradicts the fact that this angle should measure the change in angle for the perihelion in each radial period. I see everywhere that it is zero for Kepler orbits, but it doesn’t make any sense from this formula.