Backup size too small on EasyStore 2TB

I am backing up my 490GB C drive on my laptop. When I backup to my external drive it is only 473GB in size. Why is it smaller? During the backup process I select all options to backup from so I am not sure where the missing data is coming from.

The other missing could be Windows cache files, hidden files, hibernation file, page file or windows update uninstall files.

Is there any way to check and see if this is the case?

look for some free software online that can help you list all the files/hidden etc… on your computer.