The neck length varies on the same tree. I assume it's the same mechanism that produces different shaped leaves on a tree - fast growth produces more leaf definition (and longer necks?). Actually brebas, which grow quickly, tend to have longer necks than main crop figs. If you look closely at the picture of the Nordland branch, the bottom fig has a long neck and short stem, and by the time you get to the top fig on the branch the neck is shorter, and the stem longer.
Nordland figs

Top Nordland Fig

I haven't gotten any figs from the Slocan yet, so I can't say that they are the same fig at this point. But they are very similar and should be compared.