Why do Welsh Ministers stay in office so much longer than their Westminster and Holyrood colleagues?

Looking at the memberships of the two devolved governments in '99; and Westminster at around the same time, and scrolling forward to the same groups 10 years later (8 for the Scots, as obviously the change in government would have made the exercise irrelevant), there are some stark contrasts between the longevities of the ministerial teams.

For the Scots, over 8 years, just 5 out of the original 22 (22%) ministers were still in office at the end of the 8 years.

For the Welsh, 10 years on, it was 44%. And the 56% who didn't serve the full decade, all departed from the Cabinet in the first 18 months.

In Westminster, we're talking far bigger numbers, both in government and the parliamentary pool they can be drawn from. And there, for the same decade, the figure was 28 out of 108 ministers who were in office in 1999 and still in office 10 years later. 10 of whom (such as Nick Brown, Margaret Beckett, Paul Murphy and John Denham) were not in office for all of that time. So a figure of 26% or 16% depending on how you count these things.

So why does Wales manage such greater stability in government? Smaller gene pool from which to draw potential ministers lessening the push to lose under-performing ministers? Less demanding government and media environment in which to exist? More reluctant 'butcher' in charge of reshuffles? And is there a scrap of evidence that a more stable government base produces better government?