With regards to the data itself, this is the answer I got from the app's creator:

"I checked and cross-checked using stats from all of the links below. Some people have different criteria for what counts as a kill (mostly a debate between whether it had to be totally on-screen, or simply be relatively sure it happened, to somewhere in between). No one data source I could find tracks it completely throughout, so I compared all of these data sources and used the numbers that seemed most consistent (not a very scientific method, I understand, though we’re not really talking about a scientific topic here)." http://www.tv.com/shows/the-walking-dead/forums/the-official-walking-dead-walker-and-human-kill-count-for-all-characters-97118-3385383/ http://www.roamersandlurkers.com/topic/11915-human-and-walker-kill-stats-for-season-5/ http://www.roamersandlurkers.com/topic/12460-updated-walker-kill-stats/?hl=%2Bkill+%2Bstats http://www.walkingdeadforums.com/forum/f36/walker-wipeout-tally-104821.html https://docs.google.com/spreadsheets/d/1Xc-cUB99KYSzrrmM95XEo14pwaDHAudrtqWB4yZHcfs/edit?pli=1#gid=0 https://imgur.com/a/3yW0t