It’s some time ago that I used hudson/jenkins but you should have a look at the Jenkins Glossary
Unstable build: A build is unstable if it was built successfully and one or more publishers report it unstable. For example if the JUnit publisher is configured and a test fails then the build will be marked unstable.
Publisher: A publisher is part of the build process other than compilation, for example JUnit test runs. A publisher may report stable or unstable result depending on the result of its processing. For example, if a JUnit test fails, then the whole JUnit publisher may report unstable.
So I suppose you have some other build parts (apart from JUnit) that report an unstable result. Have a look at the whole build process log.