That's actually an Android system app, not a Samsung app. But it is possible that particular apps/processes are being hidden and one of these reporting services is just not including them.
I would just be more inclined to believe the Android one. After all, the garbage collection process (the process responsible for managing/cleaning RAM as necessary) would refer to the Android reports. And, the GC is constantly managing the number. Get an app that will watch the logcat (like any of
these) and you can see what the GC reports as available memory. It's called "GC_CONCURRENT"