Why such poor results with Background Erase?

[Contours]

For some reason, I thought background erase had boundary detection logic that made it easy to isolate images. But my experience over a dozen uses has left me doubting the processing and/or the methods I use.

I'm following exactly the steps show in this article:
http://support.xara.com/index.php?/K...ckground-erase

But my resulting images never have the sharp boundaries it implies I can get.

Admittedly, the range of color (mostly grays) in the image I'm currently working with is low. Nonetheless, there is a clear edge to the image. And I've had artifacts show up even on images with dramatically darker backgrounds. Up to now, I've resorted to tediously outlining an image to get a decent result.

Is there a setting or method I should try?

Thanks in advance for any suggestions.