What did they understand when they saw the wrong arrangement of the rings in the hand?

From the text:

“My rings were put on his finger; but when it came to the wedding ring,” he held out his muscular hand, “you can see for yourselves that I had struck the limit. I have not moved it since the day I was married, and it would have taken a file to get it off. I don’t know, anyhow, that I should have cared to part with it; but if I had wanted to I couldn’t. So we just had to leave that detail to take care of itself. On the other hand, I brought a bit of plaster down
and put it where I am wearing one myself at this instant. You slipped up there, Mr. Holmes, clever as you are; for if you had chanced to take off that plaster you would have found no cut underneath it."

From the positioning of the rings, the identity of the dead man is revealed to be incorrect.