contourf question

I'm notice that contourf behaves differently to contour by default in where it decides to position contours. For example, using pylab, if you try

a=tri(10)
contourf(a,0)
contour(a,1)

I'd have expected the contours to line up, but they don't. Is there a way to get contourf to place its contours at the same position as contour?

thanks,
Gary