This single photo can help explain how pecan trees create flower clusters and why some flower clusters are larger than others. Pistillate flower stalks are formed at the end of the current seasons new grow. The number of flowers produced and the ability of those flowers to remain viable is determined by the energy stored in last year's wood. Given enough plant energy, a current season's terminal will start creating pistillate flowers and continue producing new flowers up the stem until the shoot basically "runs out of gas". Flowers near the base of the flowering stalk are the strongest and are the most likely to become fertilized and produce a viable pecan. As the tree produces additional flowers, the tree gets to a point where it only has enough energy left to produce small or poorly formed pistillate flowers. These poorly formed flowers drop from the tree during the final stages of the pollination season.
Many might think the lack of nut set on this shoot was due to a lack of pollination. However, pollination has nothing to do with the abortion of the flowers on this terminal. Weak and poorly formed flowers will never produce nuts regardless of the amount of pollen flying through the air.