Time Machine Folder size and quantity of "items" less than computers?

Hello,

I just started using Time Machine and my understanding is that it backs up everything on your computer.
Well, before I started deleting things on my computer to make more room I thought I'd pull up random folders (Pictures, Music, etc.) on my computer and also the same backed up folders on the Time Machine external drive, then Cmd + i them side by side....but the Time Machine Folder sizes and number of "items" are coming up just a little short of the originals on the computer itself.
Am I missing something, literally and figuratively?

I don't know if this matters, but I did the backups because my 111GB MB had zero (that's right, zero) space left on it. So, I backed up to a 1TB external HDD.

If the back did in fact leave off some "items", how would I go about finding out what those are? I don't want to start deleting things off the computer drive until I get an answer to this question of why the Time Machine numbers are just under the computer's numbers.

Thanks,

jman995x