Not sure what's wrong with that IMO(I see your point but it's debatable). Chang was pretty good. Courier over Becker, though, that boggles belief.
Note that I only checked results in the nineties. Obviously, Becker has better career achievements and would have ranked higher had I taken 1985 as a starting point.
I chose to take 1990 as a starting point, though, as that was the year the TMS series (or Super 9 tournaments as they were called then) were introduced. The atp calendar has remained pretty much unaltered since.
And as for Jim Courier, he has had solid results on all surfaces, winning RG and the Australian open (both twice) and reaching the finals of Wimbledon and the US Open. He also won Indian Wells, Key Biscane and Rome (on clay of course).
Being able to play on all surfaces gets rewarded in the ranking system I used. Which is also why 3 time slam winner Gustavo Kuerten isn't ranked higher on the list.