> on the other hand, everybody understands that out of hundred people in the department there will be a couple of underperformers

The law of large numbers only works when the sample is unbiased. If the stack ranking was working as intended to any degree or the hiring procedures have any quality, no team will be composed of random people.

So, if that statement holds for your company (I doubt it), then both hiring managers are completely incompetent, and the rank process is completely ineffective.