Because Mayfield is a 'neurological simulation'. It's fake. [This is a known fact, but he feels the urge to recite it and perhaps to ground himself in what's supposedly reality.

But then he takes another look at Luke's ragged appearance with mounting uncertainty] ...Except it feels really real.

[Stepping out of the funhouse and into the daylight, Slugger takes in the bright colors of the carnival beneath a cracking, foggy sky.]