That is a really good question, or actually two questions. The first one @nephros already answered. The answer to the seconds question lies in the history and objectives of the three:
The Jolla store, also known as “Harbour”, provides Sailfish OS users with vetted application packages. The users who install these packages can do so without having to fear that the packages break their system. Also, Sailfish OS maintainers go to great lengths to make sure future OS versions don’t break existing applications in Harbour.
Openrepos actually predates Sailfish OS, and its main objective is just to provide application developers a place to publish their software. There is no quality control on the packages.
In addition to just publishing software packages, Chum provides application developers build service - each package is built by Sailfish OS OBS. This works well for open source applications, not so well for closed source ones.