Faulty data will be fixed in upcoming update.
Some faulty data will stay as is for now for alts. Fix for it will be deployed with the next maintenance.
Q: How accurate the info is?
For neck levels, it is 99.9% accurate, because it is using static data.
For demon data, it is not. How? Because server's are set to London timezone, which means data resets 8 hours before the server daily resets. So in reality, daily demon data takes 8 hours worth ap from a day before. Also there is a small issue with the API, Blizz APIs are not designed for day-to-day analysis, they are for static data. So if a user won't go offline for more than 24 hours, this will effect the daily data.
Eg: For say user started playing at 12 PM (GMT+0/London), and kept playing for 26 hours and gained 26k AP total. For sake of example let's say user gained 1k AP each hour. This means user should be getting 12k ap for T day, and 14k ap for T+1 day. But as stated above, API is not designed with this system in mind and does not force updating on reset moment, so pushing whole gain to next log-off.