Question: Question about implicitplot

I may be misunderstanding the documentation of implicitplot.  Can someone set me straight?

This is extracted from the implicitplot's help page:

implicitplot(-x^2 + y, x = 0 .. 2, y = 0 .. x);

The plotting range is limited to y ≤ x, as intended.  Let us verify that it does the right thing:

display(
	implicitplot(-x^2 + y, x = 0 .. 2, y = 0 .. x, color=red),
	plot(x, x=0..2, color=blue)
);

Yes, indeed it does.

Now let us try limiting the plotting range to y ≤ 1 − x². Here is what we get:

display(
	implicitplot(-x^2 + y, x = 0 .. 2, y = 0 .. 1-x^2, color=red),
	plot(1-x^2, x=0..2, color=blue)
);

I expected the red curve to lie entirely below the blue curve but it doesn't. Am I misunderstanding implicitplot?

Download worksheet: mw.mw