Perhaps .rar files have more than one setup or configuration and 7-zip does not recognize all of them? I dunno. Grasping at straws here, but I stand by what I said; it only happened a couple times for me.
I would rather post a .zip file or maybe a .7z file on here anyway, for the sake of better compatibility. On my site where I have a hundreds of bundles, they are all .zip files, since it's something (pretty much) anyone can open.