In the 3-5 year range you see different loads based on how precocious the variety is. There is a large difference from one variety to another. I have some varieties that took ten years to fruit, and some have big loads after only a couple years. Since the other variables sound similar I would guess the variety is the main issue. It could also be due to having different weather at (different) bloom times, or a bunch of other reasons.

Scott

More vigorous varieties tend to shade spur leaves in spring with rapidly growing shoots that send all their energy forward. I've come to believe that this has a strong influence on whether spurs can provide enough energy to develop fruit. Lacking adequate energy they abort. When conditions are cloudy shortly after flowering, the issue is exacerbated.

Inadequate light on spur leaves also is probably a strong contributor to biennial bearing- second to excessive cropping, of course. It is in the days immediately after petal fall where the issue is crucial- the total period being about a month.

I believe it may be useful to eliminate some of the most vigorous new wood- especially water spouts, as soon as they appear, in order to keep spur leaves sundrenched at this crucial time.

This is my own idea and only loosely connected to research based information, but it is at least a logical leap from what we do know. We know the growing shoots don't supply the fruit with energy until they've stopped growing in mid-summer. We also know that they shade the spur leaves and make them less able to harvest light and convert it to energy. And finally, we know that the crucial period for spur leaves to hold fruit and develop flowers for the following year is in the 3-4 weeks post petal fall. My conclusion appears to be obvious.

I'm not saying this is the only other issue in play here, and various varieties probably hold or abort fruit for other reasons that are more complicated.

I should have included the issue of pollination and pollinators as a crucial factor as well, but I believe this is very well known and sometimes eclipses other explanations for crop failure.