It seems it's simply "hybrid vigour" - when you cross strains, you can get the both of both pools. But there's a logical dark side - you can also get the worst of both. (In nature, hybrid vigour is a net positive, because the weaker ones are selected against...) I say "logical" because I haven't actually ever observed this in reality.

Anecdotally, Elvis Presley, Keanu Reeves and Tiger Woods seem to demonstrate this with other gene pools - but I have no idea of the statistics with respect to other actors/athletes. And there are other factors, such as non-standard features being considering more striking, the parents being strong-willed, self-determined and courageous enough to go against the norm, the other being considered so attractive and valuable to be worth the hassle, etc.