Hitting horses is kind of fickle when the rider is blocking--I've noticed this as a rider, and as a person stuck using my legs to get around. I'll often block a lance thrust at me, and hear the "clunk" sound of a successful block, but my horse will still take damage (I wont, though). Or, as OP says, I'll aim at the horse, away from what I would expect the magical block force-field area to be, but nonetheless will have my poke deflected.
It would be nice to have actually see the hitboxes of both horse and rider, as well as the "magical block force field area" that I suppose appears when a rider holds down-block. There may be overlap somewhere, or something.