All the textures use a 256 colors palette and some take advantage of the peculiar feature of the Half-Life engine of randomly tiling a set of similar textures to avoid annoying repetitive patterns.
That's why sometimes you may see more than one texture in the 'single' column.

If you're interested in using any of these textures for your own levels, just let me know. All I'd ask in return is to be credited for the (often hard) work I've done.