much of this is opinion which i have gathered from experience, reading, and discussion with others. not intended to be scientific fact.
many plants, such as fruit trees, go through a process analagous to human puberty. young plants are not capable of reproduction (flowering) because their internal biochemistry has not matured to that point. at some point, probably influenced by age, size, general health, or other factors, the plant undergoes an internal transformation (hormone change?) and begins to produce flowers.
this maturation process takes different amounts of time with various species. with annuals, such as common garden vegetables, it is quite short. with fruit trees such as avocados, it takes several years. with nut trees, it can be even longer.
scion wood is commonly taken from reproductively mature plants, and is biochemically ready to flower, given an adequate supply of nutrients. this supply base depends on the root structure of the rootstock, and the amount of leaf area in the canopy. often, a scion will begin to blossom before the supply base is adequate. this, i believe is why young grafted trees drop so much of the fruit they set. the tree discovers that its blossoms have written checks that its root system and leaf area will have trouble cashing.
i have noticed that when seedlings do begin to flower, they do not prematurely drop as many undeveloped fruit as young grafted stock does. seedlings have had a few more years to develop adequate root systems and leaf canopy while they were waiting for puberty.
again, this is mostly opinion, resulting from experience, not rigid science.