Weird indeed and occasioned by two factors: a) The creators of this "rule", forgot to distinguish between words of Germanic and French origin; b) it is difficult to foretell what the printers would decide to be "correct" in the future. Most European languages have their spelling regulated by a University, but not English. I do not know why they changed "wyrd" to "weird", although it starts with "we" and ends with "ird" as in "third", so perhaps somebody made a half-hearted attempt to regularise the spelling!. Shakespeare would not have recognised this spelling of it! The Wyrd Sisters, three in number, are the Three Fates (find them in Greek mythology), and your wyrd was your fate/destiny. The word "wyrd" is of Low Germanic origin - now try saying that sentence aloud!