HS kids in areas without their XC seasons are doing track 5ks and road 5ks, all without the pressure of weekly races. You see a kid drop a sub 15 one week and then a 16min 5k a few weeks later in an actual race on a tough course.
I’m not saying everyone that runs a 16min 5k on grass has a sub 15 in them, rather there are a lot of kids who never run sub 15 who could have done it on a track or road if that was their only goal in the fall.