Horse is more than likely shortlegged on left front, making him reach down further on each footfall. Short of like if you were wearing a high heel on one foot and flat shoe on other, you will be out of balance, and end up sore backed as a result.
I've seen horses be as much as 1/4 inch uneven.
I am not talking about hoof length either, but from where front leg ties into chest to the coronet band. Too many will only look at hoof length being equal, and while that is fine and dandy, if one leg is actually shorter? Then horse is coming down unevenly.
Horses, nor for that matter anything living, are rarely, if ever symmetrical. But measure the length of the front legs, NOT including hooves, and see if that is problem. It could also be back legs, but would bet more on fronts.
Shoer can correct with leather pads, as needed.
And could also be something in way horse walks? Swinging one leg out, or twisting funny? Need to video, front and back, at walk and trot in hand, and again under saddle. Rider could also be putting horse out of balance too.
Horses make me a better person.