The mileages for any long trail change all the time; often minor changes, sometimes significant ones, such as some major PCT re-routing in SoCal. This can come about based on physical trail changes on the ground or from more accurate (?) measurements.
If you want an up-to-date measurement in a way that's useful for trip planning, the latest guidebooks will likely have the latest official mileage. One option is to join ALDHA for $10 and from that have access to the pdf version of their guide, the 'Companion'. http://www.aldha.org/companion.html
You can look at this at home on a larger screen, and/or carry it with you on a smartphone. I did the latter on my AT trips, as a backup to my main guide ---I liked using AWOL's "A.T. Guide". You can get this for $16, or $9 as a pdf download. http://www.shop.theatguide.com/Books_c3.htm
But without studying the issue, I would assume that mileages given on the ATC website are good.