Hi Jeff,
Like most ROs I'm sure, in the old days I used to either count contours, or just take the top height of a climb and subtract the bottom, then add them all up.
For a more recently created race (Castle Carr) I used mapping software (Tracklogs) and got it to tell me the climb, and went round with a GPS to confirm.
But for the existing races, the climb will just stay set as what it always was. Perhaps some ROs will remeasure digitally, but I wouldn't have thought many would.
Regarding distance being more accurate - its a lot easier to measure distance on a map than count contours, so I guess that might be the reason?
So all pretty much what you surmised. There's no cunning formula!