Graeme,If you don't have the Altavista engine itself checked in your admin, I would imagine those results are being returned from one of the other engines that you do have selected. I don't know which engines feed which anymore, but that's my guess.
In other words, maybe instead of engine --> final end URL
you may be seeing
engine --> AltaVista result fed to engine
You might want to try the engines one at a time until you find out which one is creating those.
That may be wrong, it's just a guess, as I haven't run into this before.
Jeff