Many reasons. Nitrogen gas storage, dwarf rootstock related sap restrictions, sugary varieties with all their flavour in essences that evaporate within two weeks, water system delivered petroleum-based fertilizers, excess size, continually re-created juvenile wood, and hormone manipulation, such as below:

Royal Gala in Misery, Bella Vista

These leaves have survived minus 11 Celsius. Talk about being out of season!

Summer pruning to force light onto the apples is making use here of the midsummer cell differentiation period, which should be laying down fruit buds in high hormonal areas and leaf buds in areas requiring branch renewal. Here, encouraged by high nutrient levels and cunning timing, the poor tree is confused by it all and threw out a spring shoot with flowers, instead of a replacement branch for the one that was cut off. The flowers did not develop normally, and came to nothing. They’re usually sterile when this happens. So, now you know, too.