My guess is that the assignment is based on the old myth that some trees have large taproot systems. The old drawing where the rootsystem is anolgouse to the crown dimentions. Some trees will have sinker roots but the vast majority of all roots are in the first foot of soil. this is where all the H2O and O2 are found. they are the biggest limiting factors to root colonization of soil. But as anyone with a cracked sewer lateral will atest to, roots will grow anywhere the environment will suport them.