Very close Loramin.
Many ZEMs were gathered manually from late 1999 and onwards by working out the math from the XP gain #s as people killed mobs. But a comprehensive list was included into the client memory in 2003-ish and the ShowEQ guys used a debugger to dump the entire list of values circa that moment in time. That list from 2003 is the source of all the 'ZEM lists' we have today, basically. That's also why ZEMs are so different from classic.
Here is the thread on the ShowEQ forums where they started compiling the comprehensive list for the first time.
http://www.showeq.net/forums/archive...hp/t-3686.html
And another with more info on how it was done:
http://www.showeq.net/forums/archive...hp/t-3658.html
The reason these ZEMs (especially for the low level dungeons) are wrong for our era is because of this patch in early 2002 which raised them:
https://www.project1999.com/forums/s...8&postcount=30
So that's why you have people in the ShowEQ thread all like "OMG crushbone zem is 160? I never knew!", because the knowledge that it used to be standard 80 had been lost to time:
https://www.project1999.com/forums/s...4&postcount=14