I would mostly attribute it to what the pilots are likely to notice - 500' AGL is a standard call-out on landing approach (not that light aircraft pilots necess

I would mostly attribute it to what the pilots are likely to notice - 500' AGL is a standard call-out on landing approach (not that light aircraft pilots necessary 'call it out' but their Foreflight or whatever very well may have), and the airport in question is actually uncontrolled so "one mile final" would be a common point to make a radio call. So it's probably that it happened somewhere near those numbers, and they were the references the pilot readily knew of.