Storage used by applications doesn't match between folder info and system report
Not all applications are stored in /Applications. That's the default install folder for most applications, but it doesn't include all applications. In fact, since applications are really just bundles of files, most applications can be run from anywhere. 

To demonstrate this, take a look in System Profiler at Software > Applications. You'll see lots of software that isn't in /Applications



With that said, using this as a method to eliminate large applications probably isn't the most efficient or easy. You'd be better off using a disk scanner like DaisyDisk, Disk Inventory X, or some other alternative.