No idea, don't really care much either.
I know some individual ride numbers. If I ride from here out across the levels it'll be about 3 ft/mile for the whole ride. From here to the top of the Blackdowns or Quantocks then about 200ft/mile one way. Here to Bridgewater and back on the canal, 1 ft/mile if you're lucky.
I can understand why an individual hill is of interest, and just about for the aggregated uphills of an individual ride, but can't really see what's important about a long term figure. Please tell.