Sales don't always equal success. Plus, literally everyones album sales decline.
TVR era got Janet 1 #1 hit
AFY era got Janet 2 #1 hits
It's likely, with the previous success, that Damita Jo might've got Janet one more #1 Hot 100 hit. Like @janet.1814 says, things like a world tour and the Superbowl shouldve made Janets exposure on the charts even bigger (which results in higher sales)