I'm open to good arguments from Anker. If it were say 5A total across 2 3A max, then it would only be a problem if both devices capable of drawing 3A, which is going to be rarer than if just one device was wanting 3A and the other less than 3A.
It is rarer that all devices need maximum power the more ports you add.
We get this same point on most of Anker's USB chargers where the max combined power is less than the sum of the max of each port, e.g. Powerport5 is 8A across 5 ports but 2.4A max per port, where the chances all 5 ports have devices all needing 2.4A is rarer.