Here's a question, one that has always puzzled me.
At home on my allotment I can only plant potatoes in the same spot with a three-year interval.
On the Canary Islands where pots are a staple diet, potatoes are harvested three times a year from the same plot and used again the next year and so on.
How come?
Maybe they haven't read the same books as you?
do they not get blight over there? if disease is not prevalent, maybe they don't see the need to "rotate"
Things like eelworm would still be a problem. Monoculture is never a good idea. A two year gap is usually all mine get, three group rotation.. :-\
Are they doing commercial or subsistence farming? The reason would depend on what they're doing and why. It's even possible, since they're islands, that some diseases have never arrived there. Blight didn't get this far till the 1830's, after all, and if we had more sensible import policies, maybe some of the diseases would never have arrived at all.
These are mainly family plots, clinging to each hillside. Even restaurants have their own plots and unheedingly plant three times a year.
Blight is clearly not an issue there.
Not only that, but they probably don't have much land if they're family plots. So they end up having to grow the same crops time and again.