Percentiles are not helpful, because at least in my home country what degrades the maximum potential speed is the influence that parallel copper cables have on

Percentiles are not helpful, because at least in my home country what degrades the maximum potential speed is the influence that parallel copper cables have on each other. So my internet speed depends on how much my neighbours are using the internet. It's really really difficult to predict this statistically. OTOH, there's a guaranteed minimum speed the ISP has to deliver.