Why do I get the boundary of the image space with Gray_level_image?

[stla]

Hello,

I take a regular grid of an iso-oriented cuboid centered at 0 and for each point (x,y,z) of this grid, I calculate x² + y² + z². I save this stuff in an ANALYZE file and I run Gray_level_image with some isolevel r². Then I get the centered ball of radius r as expected but I also get the boundary of the cuboid:

Do you have an idea why?

[stla]

Get it. I replaced the voxel (the 3D array of values) with 2r² - voxel, and I got the ball only.