I don't put a whole lot of though into the "why not" of a horse not coming.
I do the night feed at the barn and every night I go out to the pasture and yell for the horses. Same time, same food.... sometimes they come, sometimes they don't. I yell louder, call them names, threaten to turn them into Alpo and then they'll sloooowly wander on up and then practically trample me because OMG, she's here with lead ropes and that means FOOD!
Who knows... I just lump it all in with things like the pony who never seems to figure out that to go eat dinner he has to walk around the gate and into his stall. Nope, gets stuck on the backside of the gate every... single... night.
Now my horse is ALWAYS ready to go it. I never call him, he's jumping up and down at the gate because not only is dinner ready, all the other horses are inside and he's ALONE!! (he goes in last).