I suppose you are right 172 driver. The biggest problem with contaminated runways is that when you are in dynamic situation i.e it’s starting to snow or raining heavily it can be hard to know when the runway is contaminated and what the braking actions are. It’s not not like there is someone measuring the contaminant depth or checking the braking action every few seconds. Pilot, reports are not reliable either. Maybe the SWA pilots, thought that the braking action was better than it truly was.
I don’t know much about performance as a subject, other that it is kind of vague.