could just be that , and taking into consideration--white mulbs supposedly live 200 years or more, while red mulbs supposedly live to 75. The longer-lived alba seedling/sucker gave the old rubra clone a headstart, but this gradually faded with the mechanical girdle, resulting in the rubra acting more and more its age(and on its own adventitious roots), and less and less of its constricted alba roots.
have observed that when planting trees from seed, vertical growth/trunk formation generally supersedes bushy/branchy growth habits. Only after a few years(or decades for long-gestation trees) will the bushy/branchy canopy formation be a priority. Once the canopy of maturity(specific to the species) is formed, air layering the branches will generally not revert the air-layered clone into prioritizing trunk formation, but will retain the bushy growth habit. Growing avocados, citrus, or even tomatoes from seed are some examples. Rapid growth to form a solid trunk is prioritized in youth, but once the specimen has matured with fruit-bearing branches, air-layering the matured branches will not reset them into juveniles, and will continue to behave like they would as if they were still growing on the canopy, hence the dwarfing effect--unless grown in light-deprived conditions.
many citrus(grapefruit/calamondins, etc), when grown from seed form thorny trunks and main branches, but are practically thornless at the thinner branches of the canopies of mature/fruit-bearing trees. One will never see them as spiny specimens at lowe's because the old mature clone is grafted low to a young seedling. The seedling rootstock has been decapitated and stopped from forming its spines, while the old clone scion seems unaffected by the juvenility of the rootstock it has been grafted to.
some species, such as jujube scions, will be affected by its relative distance to ground level, and will resume thorny growth, but will continue to act its age by bearing flowers, often on the same year they have been grafted. A stem from a 1 year old juju seedling will not bear flowers when grafted to an old juju tree, just as a stem from a 1 year old grapefruit seedling will not produce flowers when grafted to an older citrus rootstoc.