The Mac-installed zip utility app that comes standard on mac machines adds some odd files [for some odd reason] -- that screws with PC users wanted to read mac-generated zips or CBZs.
For anybody curious as to what's going on here (which I expect are few, with a solution), the extra files are what Apple calls a "resource fork." It explains to your Mac what kind of file it is and who's supposed to be responsible for opening it, among other things.
The UNIX approach was historically to analyze the file to guess what kind it is, which can be slow and error-prone, obviously. The Windows and web approach (from CP/M, and possibly earlier) is to work with file extensions, which is arbitrary and too easily fouled by the user. So from the early Mac days, the system creates an extra file containing all this that the user can't see or touch.
The downside is that you're left with a choice when archiving: Do you include the files and confuse your Windows and Linux friends, or do you discard the files and confuse your Mac friends? So different programs make different choices, and we all end up confused.