I really think that a horse coming when called is more about the relationship that you have with the horse than anything that's trained or taught.
I've always boarded my horse, so I don't feed. I also don't treat my horse, other than scratches on his neck. And yet he always comes to me when I call him.
Also depends on how nosy your horse is
Selena will come up to anyone and their brother who shows up at the fence...
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.