Yes. Generally such restrictions come into the culture slowly. I'm assuming, with some knowledge of these cultures, that these restrictions were likely pre-startup. Many of the aspects that are adopted by a religion actually originated long before it, sometimes thousands of years before it, as adaptation to local conditions. While you and I don't see a reason for that in this case there may have been some other intertribal strife that had caused that. And besides, 1,400 years is close enough.
The bigger point, which you're missing with the arguing over generational counting, is that such a change could cause different cultures to fail to understand each other on a very fundamental level. Lets not quibble over a few generations but rather try and think about why and how something occurs so we can better understand it and solve the problems of cross cultural misunderstanding.