Number of iterations of the CG method

[USUARIO INICIAL]

I have a conjugate gradient implemented in C without Petsc and when I
solve the linear system of my problem (an elliptic pde) it converges
with something about 270 iterations. But with Petsc (ksp_type cg
pc_type none) the same linear system converges with 510 iterations.

Do someone have any idea why is that happenning?

I already checked the matrix and they are equal. The results are quite
close and I set the same tolerance to both methods (setting rtol on
petsc to a very small number, to use only atol). And besides the
number of iterations, the time spent by petsc is not much more than
the other method.

Thanks,
Ronan

I'm sorry, I dont know english very well, I did the best I could.