Time machine missing backups?
It is a nasty bug of OSX 10.9 and 10.10 . Some workaround is described here: Time Machine is skipping files and folders. How to fix? by Pete.

... I see you are running 10.7.5. So maybe Apple ruined Time Machine already with Lion. I personally never had the issue with 10.6.8, then directly upgraded to 10.9 and then later discovered missing files in my backups. So you can confirm if the above workaround also helps with OSX 10.7.5 Lion.