Why not ground a relay internally? It's a production shortcut. You eliminate the need for a 4th terminal. Internally grounded solenoids are pretty common for starter relays when the triggering wire is hot, but not with a switch that grounds the coil. But in the case of a horn relay, it's like consolidating the #30 and #86 terminals in your 4 pin relay diagram in to one, and the wires separate inside.
