I was sitting in the movie theatre (Smurfs, don't bother, it's naff) and I had an epiphany. It's not the leading, it's probably the font size! Somewhere in that paragraph will be an invisible space with a different font size.
To check, hit Cmd A to select all the text in the box. Then look at the size field. If there's a number in the box, I'm wrong. But if there's a blank space, I'm right.