To reproduce just set up mediawiki with a fresh database, login with the user specified at installation time and go to Special:CreateAccount. You'll see this:
Surprisingly this even happens when you do a few edits with the account specified at login time. To me, it even still happened after creating two test accounts and doing some small test edits with those. As you can see there are 13 edits (by 3 users, one of which was created at setup) but it still shows -1 recent contributors.