Ok... what size is the hard drive?
Control click the MacintoshHD icon on your Desktop then click Get Info.
What is the capacity?
Thats extremely common for the updated available GB to post a few mins, or even 20 + mins later.
Ive seen it 100 times.
Especially if you deleted, say 10gigs of 1000s of files, instead of 10gigs of just a few huge files.