The connection finder prioritizes the shortest path first, then chooses to display the one that has the longest stretches of blood relations in it. I took a peek at your connection to Billy and noticed that starting with you and going through your Marshik side, it's 9 degrees of blood relatives into the connection before it switches to a marriage. Then through your McCarty side, it's only 8 degrees before it makes the switch.
Also, on Billy's end, the McCarty side of his tree isn't as dense as the connections through his stepfather.