Remember hearing in the past that Charles Hicks and Ky Robinson used to design their own training plan at Stanford. We can assume that the standard and quality of training was higher under Brosnan at NP than at Stanford just by looking at the times.
Lets not forget that Cole Sprout was one of the most impressive HS runners ever, and he hasn't been able to get anywhere near the likes of Nico (where would he be if he went to NAU?). The twins were objectively quicker than Nico in HS also. Nico and Colin developed extremely well at NAU, Aaron not so much, but I don't think he takes it seriously anyways. I think L&L would be significantly quicker if they committed to NAU.
I'm guessing here, but the twins seem more like the "frat" type, I imagine they are also enjoying college life (maybe a bit too much), the workload at Stanford is also higher. When you couple that with training that isn't stimulating improvement, you get a plateau.
Also just want to say that you guys basing this difference entirely on 'talent' are stupid.