I think mathematical parity has been around forever, but I don't disagree that right now it's impressive that 10 of 15 teams in the AL are at .500 or better. In the NL it's just 8 of 15 teams.
A footnote to the above is that Miami, Tampa, and Baltimore team salaries are ranked 22d, 28th, and 29th in MLB and all have solid winning records. Indeed, all three have better records than the top four MLB team salaries: Mets, Yankees, Padres, and Phillies.