I've now had a good chance to chat with a meteorologist - the chap in question used to be one of the flying observers on Snoopy and has also been a forecaster instructor at the Met Office college. In short, he knows sheds more than any of us about winds and weather.
His view is that there's no meteorological reason which explains what's being seen.
The more I think about this, the more I think that the phenomenon as seen is a function of the algorithms being used in certain common nav kit, and absolutely nothing to do with aircraft performance or meteorology. Cap'n Pitt Bull's No.2 in other words.
G