Macintosh HD Internal Hard-drive Issue! Helps me peeps?

Im pretty sure this issue has been reported before, but mine has a little twist to it. I just want to make mines clear!!

- My Macintosh HD says its got 51.66 GB available in "finder"

- "Disk Utility" is telling me there is 14.96 GB available

I am certainly sure I actually have more than 51.66 GB on my mac

- Let me start here -

- I recently downloaded a 20 GB file

- Before downloading it I'm sure I had atleat 70 GB available on Macintosh HD

- After downloading it, I moved the file to my External Hard-drive

- By that, meaning, I should have "restored" the 20 GB back to my Macintosh HD right?

- But now, its still saying I have 51.66 GB available

- And Disk Utility says I have 14.96 GB available

Where have i gone wrong guys?

Can anyone of you help me with this issue?

I Have a Macbook Pro with Retina Display, OS X Mountain Lion (10.8.4) and just purchaced it recently before I started university!!

It would be greatful to hear your feedback as I shall help you oneday too!!