said by Warzau:
Again who knows how long it's been like this.
It's been that way since someone over-torqued those nuts. Looking at how deformed the washer is on the other side, I'm amazed that portion hasn't cracked yet!
It probably won't leak so long as no one leans on or otherwise applies any force to the tank. Seeing as to how there's no way to guarantee that won't happen, you really should replace the tank.
Looking closely at the pictures again, it looks like the whole thing may be assembled incorrectly. There should be rubber washers that go between the metal washer and the bottom of the toilet bowl lip. Am I seeing the rubber washer stuffed between the tank and the bowl.. ??
Edit: to add pic. In other words, the piece I highlighted in red belongs where I drew the blue line.