The first thing to understand is that that screenshot does not show everything, just certain types of your data use.
The big omission is that it doesn't show the space used by the ROM and system firmware. I don't know how much space those take on this specific phone, but I would guess that at most you have 10GB available out of that 16GB, with the rest being used by the system partitions.
Then the next question is what does that screen mean by "SD card"? Does it mean an actual removable SD card, or the emulated "internal sd card" (which appears as /sdcard to a file browser)? Because that second one is your internal storage emulating an SD card, and so if that's what the storage screen shows that's 7.4GB of your perhaps 10GB used right there. Throw in apps and app data, which your screen shows as 2.4 GB, and you've used almost 9GB. And don't forget that I'm only guessing how much space Samsung leave for you and how much they use for the system, it could be less than 10GB. If you want to know for sure, there's a really tiny app called "Storage Truth" written by one of our moderators, which you can find in the Play Store. That will tell you how much space is actually available for user files and data and how much is used by the system.
If that last item really is the removable card then yes, there's stuff to account for. But in that case consider that that screen is only showing usage that it can identify as falling into a small number of categories - anything else just isn't shown there. A better estimate would come from using a file explorer to see what is in your internal storage. Or try an app called DiskUsage, which can show a graphical view of how your storage is actually used.