I was taught that the 12 inches was about the shortest length of flex, entering a box, that could be bent in a 90. Consider the amount of flex that goes straight in to a flex connector, plus the flex that is needed to bend a 90, then strapping it..roughly 12 inches.
Course, this rationale doesn't work for larger diameter flex. The larger sizes need about 12 inches just to straighten an offset out before strapping it.
Just to add to the thread: Is the 12 inches measured ALONG the flex (or MC, AC, or NM), or just the distance from the box connection to the support point?