Although genetics do play a very large part in running, I think there are also many environmental factors that can separate the 15 minute 5Kers from the 18 and even 20 minute 5Kers doing the same training.
So far, I've anecdotally noticed two major things:
1. Build/Bodyfat
I think someone's build/bodyfat is one of the biggest determinants for how fast a new runner will be in a coming season. The new runners who I've seen that are lean and skinny/midweight have tended to start off with much more success than other people.
2. Sports history/Aerobic base
Swimmers, Soccer players, and Tennis players similarly start off with lots of success as a runner. I only put this second because people who have more sports history and have a stronger aerobic base are already fairly lean.
And here are a few people who I've specifically noticed. I have rarely seen anybody break this pattern when just keeping these two factors, but that may just be because this is a small sample size and I'm mostly using personal anecdotes.
Person A: Very active and very lean, in-shape starting his first XC season. Though, for some reason, he is very slow. By the end of the season, he has dropped nearly 10 minutes off of his 5K and finishes with a high 16-minute 5K
Person B: Soccer player with a soccer player's build and a 7-minute+ mile comes into track, breaks 5 minutes in the same season. (Basically like another Person A)
Person C: Very skinny and very lean soccer player comes into track his first year, runs 2:45 and drops to 2:30. Next track season, he starts off with a 2:30 and drops under 2:10.
Person D: Person who mostly stayed inside, very academically gifted, just went outside occasionally to play with his friends (Tag, walking around, soccer. Inconsistent, 3 times a week max). Guy even without much exercise is somehow very lean through his diet. In his first track season, he drops from 7 minutes to under 5:10.
Person E: Person is very lean, don't know his athletic background. Is slow throughout all of his XC season, but comes into track and runs a 52 second 400m and does high, long, and triple jump.
There are a few other people (mostly Freshmen and undertrained people) who I've seen be lean and slow, but I haven't seen anybody that is lean with a good aerobic base come out and be slow. This is what I've noticed after 2 years with a big team. The training is mostly low mileage high intensity on-season (20-35mpw XC/Distance Track), mileage in the off-season (40-50mpw MAX)