Minecraft wiki entry for Skeleton_horse_farming, associated text: "Originally, we wanted lightning hitting the lightning rod to be identical in any way to the normal lightning strikes – but that proved to have some unexpected consequences. I was working on lightning deoxidizing copper blocks so I built a nice little copper roof with a lightning rod on it and left my game running while I went out for lunch. When I came back I found many skeleton horse traps waiting for me on my roof! We felt skeleton horse traps should be a rare and exciting occurrence and constantly finding them on your roof might take the mystery out of them. That's why we decided to make lightning striking lightning rods never summon a skeleton horse trap." - NIR "ULRAF" VAKNIN