This is a known issue:

http://tools.cisco.com/Support/BugToolKit/search/getBugDetails.do?method=fetchBugDetails&bugId=CSCsq53214

To get them in sync again you will need to reboot the sensor. For some reason the health monitoring code on the sensor (which is responsible for the dashboard information) is not properly detecting the installation of the signature update. A reboot forces a restart of the health monitoring code and it then properly reads in the current sig update level.

Do understand, however, that the restart is just a temporary fix and it may still happen again.

This does not always happen. I am not sure if developers know yet what situation causes this to happen.

If this issue consitently recreates itself on your sensor, then you might try calling the TAC. Your sensor might help development in debugging the issue.