I was told by an old time farrier, one who apprenticed for three years to become a journeyman, did five years as JM then became a master, that two different front feet comes from grazing in the same position, so the forward foot grows long toe low heel, the backward one becomes somewhat clubby. Can and should be corrected from foal age on, but most breeders don't give a darn.
I've seen attempts to fix it on an adult horse...very ugly outcome. Horse is pasture pet and lame.
You did right in rasping the heel a bit to help spread the frog. Should be done also on the flat one, since an underrun heel actually grows forward instead of down. Lots of Farriers don't know that and don't trim the heel "because it's too low already".
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