The package info for both indicate that they come from the same website. It seems strange and inconsistent with what is normally expected. For example, there are not separate packages for gzip and gunzip or bzip2 and bunzip2.
1. They are represented as distinct programs on the website.
2. They are developed independently, they even have different version numbers.
3. Although the binaries are very small, some people only need one of them. I never create zip files, but I have to unzip them often.
4. They seem to have different dependencies.