Consider, for example, a swim meet. If you look at the competitors in prelims, you'll see a variety of body types. By the semifinals, you'll see a smaller variety, and by the finals, you'll likely see a field of competitors with similar builds. Most look at that and assume a build like that of those in the finals is the result of an intense swimming regiment, when actually it's that body type that enabled swimming success. The entire field of competitors may have put in similar levels of effort, but the physiological advantages genetically granted to the better swimmers is what resulted in their relative success.