How do you claim on one hand that YC can materially improve your company, and on the other judge the company for not being improved after going through another

How do you claim on one hand that YC can materially improve your company, and on the other judge the company for not being improved after going through another accelerator? Because they're not claiming credit for the success for their alumni, and they are also giving other accelerators the benefit of the doubt that they are good too, and acknowledging that the primary factor is the founders themselves. If they don't thrive after going through another accelerator than why would YC be any different? I don't see the conflict here at all.