Thanks for the feedback, Mike. If you want gorgeous scenery, NZ is the place to go. As Tekapo is a glacial lake, that's actually the colour of the water - stunning.
I have to say that if that's what the effect is, then it's no wonder I haven't seen it before. I rarely pixel-peep and look for the overall effect of the photo rather than poring over details - and I tend to use the wide end for landscapes anyway. Yes, the trees and shore on the right were closer than the left, which is the direction that the road in the distance goes away into. Actual point of focus? No real idea, although it may even have been the bushes in the lake itself, as they're well-defined. I tend to pick a feature to focus on, compose, focus and re-compose, so the centre or very distance is rarely the point of focus for me.
As for the foreground bush being in focus: I have no idea how far away it was, but I was standing on a bank above it. My trusty DoF calculator says that at 40mm and f7.1, everything from 13 feet six inches to infinity should be in focus, and if you laid me end to end twice (oooh matron!) and added a tiny bit, I'm estimating that's at least as far away that the bush was from me, so I'm not surprised that the twigs are well defined. If I did focus on the bushes in the lake rather than on the mountains (or even closer - and I can't remember, so just guessing here), then the point of closest focus per DoF could be even closer such that at f7.1, infinity on the lens might not be covered by the DoF so that the most distant mountain (the one on the very far left) would be out of focus, which it is, slightly. So many variables!
Maybe it was the pixie dust interfering with field uniformity! At the end of the day, all lenses of all makes (especially wider ones) have field curvature to some degree, even those into four or even five figures maybe. Some have other foibles such as being prone to flare or chromatic aberration, and many people here say that the 15mm Limited has poor edge sharpness. For me,
if there is a small effect with the 20-40 then in the general scheme of things with this lens it really doesn't bother me, so I'm sure that I will indeed un-notice it!