1) The fact that biological processes tend not to have hard limits and "kill switches" at those limits, but rather feedback loops which increase or decrease in efficacy due to a number of factors, which are themselves factors for further downstream processes

2) The tendency towards physiological plasticity to facilitate functional homeostasis (e.g., there was a case of an athlete whose smaller blood vessels enlarged in order to maintain blood flow following an embolism; this was disabled athlete who walked on her hands in lieu of her non-functional legs)

I think what is more likely is that that there is a level of a given quality which your genes will have you tend toward (with environment determining how much you will overshoot or undershoot it), but also that the human form is very adaptable and will often respond to pressures by routing around lower-functioning systems, which may require unconventional support but also create unconventional and exceptional achievement (the aforementioned athlete is a paralympian with arm-based muscular strength and endurance - a neurological matter - which outpaces most abled athletes).

You've got far more food than you need. You can't get any taller -- what do you do with it? Should you eat it at all?

How resilient should you be to less-than-ideal input? Say it's unseasonably cold for a few months. Should you get shorter, or is it better to just give up and die?

You don't have enough food. You're not in any danger of starving, but you're not going to reach maximum height, IQ, _and_ athleticism. Which trait(s) should be lowered? By how much?

Genetics doesn't just specify the maximum outcome. It specifies the entire set of possibilities, and how those possibilities are reached.