Weird Mac Free Space Increase

There seems to be something that's not right with my 2011 MBA. I think every time I install an update that requires a restart, my free space on my MBA SSD will increase.

Once it was around 22GB left and after installing an update I have 24GB free space.

Recently, after installing OS X 10.8.5 and iTunes 11.1.1 (not that it matters), and after rebooting, my hard disk space increased to 26GB.

What could be happening? I'm worried that I'm losing files for no reason. (Trash in the trashcan is still there btw)