... On the way, Harry filled Ron in on everything Sirius had told him about Karkaroff. Though shocked at first to hear that Karkaroff had been a Death Eater, by the time they entered the Owlery Ron was saying that they ought to have suspected it all along.
“Fits, doesn't it?” he said. “Remember what Malfoy said on the train, about his dad being friends with Karkaroff? Now we know where they knew each other. They were probably running around in masks together at the World Cup.…
I figure "fits" here means something like: "it fits for the situation" or "that makes sense". But I don't know if my understanding is correct? Is it short for something?