the <buildNumber> always be 1 on maven-metadata.xml

We have setup a project build on cruise control, whose version is SNAPSHOT (in my case is 70-SNAPSHOT), however, the build result files look fine, but the build number always be 1, what is the problem ?

You may compare the 2 pic, you will see the difference. the right pic is what we expect. it has the build number 54,55, 56, 57,,,,
while the left pic's build number always be 1

in the maven-metadata.xml the buildNumber always 1, no matter how many times you build it.
<buildNumber>1</buildNumber>

<metadata>
<groupId>ubhsc</groupId>
<artifactId>sc</artifactId>
<version>70-SNAPSHOT</version>

• <versioning>
• <snapshot>
  <timestamp>20090320.170754</timestamp>
  <buildNumber>1</buildNumber>
  </snapshot>
  <lastUpdated>20090320170754</lastUpdated>
  </versioning>
  </metadata>