AC Threshold :
What do I make of this ?
The number above is the AC for which your attack will be doing the same damage with and without Great Weapon Master / Sharpshooter.
If the thing you're hitting has a lower AC than that, you should be using the feat to maximize damage.
The "Super advantage" calculation is slightly different and thus shows a rounded AC. This is the maximal AC you can "safely" target WITH the feat.