Does anyone know what determines the number of shallots that develop from a set?

In the past I have had sets (of the same variety, in the same season) that produced between four and twelve bulbs each. I didn't treat them differently as far as I know. This year, so far, the sets are splittling into very small numbers - there's a while to go, but it looks as if the most will be four or five.

I imagine that some varieties have a heavier yield than others, and I guess that the growing conditions are a contributing factor... but in what way? What do I aim for to get the best possible yield per set?

I would be interested in anyone's ideas.

Thanks

CC