[petsc-users] Very low CFL number for PETSC's GS

I wanted to compare my own GS and the one of PETSC's. I used KSPRICHARDSON with PCSOR to obtain GS. I tested my GS with CFL=40 and solved Ax=b problem successfully and fast. However, PETSC failed to solve at CFL=40 and it gives an answer only for very low CFL numbers such as 0.1. Of course, the convergence was very slow. My question is that if A and b and the implementations are the same why PETSC fails to solve with same CFL number as for my GS solver? PS1: My GS solver is a plain GS, no fancy stuff. PS2: A is a block matrix (MATSEQBAIJ) for PETSC. Also, the process is sequential.