I think a lot of this has to do with the rules in WPX: if you don't get the serial number right you lose the qso. That means you really have to work hard to work the weak stations. You can often guess a callsign partially from a callsign database (master.dta), but not a serial number.
That rule has existed the whole time (and far all contests AFAIK, maybe some "QSO parties" excepted) but for a long time in the 90's and 2000's few of the big guns especially in WPX ever seemed to concentrate on correct exchanges both way to the point they might let their rate drop by a few QSO's an hour.
One of my favorite parts of the ARRL Sweepstakes writeups, is they single out and congratulate stations that had a busted call/busted exchange rate of less than 1%. And the sweepstakes exchange is one of the most detailed there is. At least one of the big scorers in the SS last year had an error rate of 0.1%. That is extremely impressive. I personally can imagine getting into the 1% club someday, but 0.1%? Wow! That's a good op! I'm guessing extensive traffic handling experience.