Alstroemeria and Bomarea are relatives, they share a common ancestor.
Bomarea stems climb on other plants and if you look closely they actually
twist their leaves so that the leaves face the sun (I went out to look at
my plant just now and it did exactly that!). As a result, some of the
leaves will be resupinate (twisted), others will be non-resupinate
(regular). This may be the case with Alstroemeria too, but they don't climb
and the chances of them falling over isn't too great. But if that did
happen, maybe they too can untwist their leaves (experiment with your
plants and see). So perhaps somewhere in the common ancestor of Bomarea and
Alstroemeria, the leaves acquire this twisting mechanism and Bomarea kept
it, some Alstroemeria kept it, and others lost it as Jane pointed out. It
makes sense. If you don't use it, you loose it.

Nhu