I only checked this in the German Wikipedia, but I bet it happens for other wikis as well.
Some users, let's pick .snoopybot. have contributions but their editcounter is set to 0. (Same for Maintenance_script or OgreBot)
I checked the replica DB on WMLabs:
select user_id, user_name, user_editcount from user WHERE user_name='.snoopybot.' limit 1; -> 0
select * from revision WHERE rev_timestamp>20070101000000 AND rev_user =425305 limit 4; -> first 4 are shown.
I can't execute a SQL query that shows me all differences between these tables for all users, because it is a very slow query, but I hope you have better tools to find other users with an invalid editcount.