Final answer:
The phenomenon described is known as regression towards the mean.
Step-by-step explanation:
The phenomenon described, where extremely tall parents have tall children who tend to be a bit shorter, is known as regression towards the mean. It is a concept observed in genetics and inheritance. In this case, the extreme height of the parents is likely due to a combination of multiple genetic factors, and when those factors are inherited by the children, they tend to even out towards the average height of the population.