Didn't know that.
Still, if you look at Djokovic's points breakdown his worst 1000 tournament (IW 90pts) is worse than his fourth 500 tournament (Rdam 180pts) but the 90 points are counted and the 4th 500 is not.
It's not the worst 1000 tournament. All 8 except for Monte-Carlo are counted irrespective of other results. For Monte Carlo, since it's optional, it replaces your worst 500 result.
So Since he got a 600 in Monte Carlo alst year, better than his 180 at Rotterdam, he can count that as his fourth 500 tournament.