In general, the AM maps are useful to show where a broadly listenable signal is placed. In reality, in noisy metros, the useful coverage is about 20% inside the innermost contour. Beyond that point reception will not be reliable / enjoyable / possible due to noise and interference. So the second contour would be "you can probably hear it if you try a bit, but it may be noisy or otherwise not a clean signal. And the outermost contour is basically on where all bets are off... you may get the station, it may be blocked by other signals, it may be covered with noise and it may be at different strengths on a seasonal basis.
85% of fixed location (not in cars) listening to AM occurs well inside that innermost circle or contour.
Remember, the radio-locator site used to have a notice that said "for amusement purposes only" because the maps are mathematically calculated, not based on actual signal measurements.