Or the fit between employee and manager. I've come into many teams where the employee on a PIP went to being one of my best performers while those I was given the ravest reviews for were just mediocre under me. Or even just cultural. I had to change how I managed/my expectations as I moved positions around the country or when offshore teams were brought on.

I agree with your shocking premise that people are not machines and expand it to include that they are also not numbers in a spreadsheet or HR system.