Hype, pure and simple.
I am attempting to breed Miniature Horses, I shall always do so, I have no desire to breed Ponies BUT, I have been going for over 25 years and I am not there yet.
Whatever is said about height there is a lot more to it than that as if you made a TB 13.2hh it would not look like any pony I know- it would still be a horse in shape, not a Pony.
Pony shape is the "primitive" shape- big head, short neck etc Barrel body, short fat hairy legs, etc etc.
Horse shape is slender neck, small head long legs in relation to body size, slender, not so prone to body fat (when fed a normal diet) etc etc.
It has been described as a Horse having a infantile outline and a Pony having an adult outline.
And so what if your Shetland does not fit in with this- I can see little or no difference between the American "Shetland" and a Miniature Horse anyway- in fact since the US Shetland is so much older a breed some of them are streets ahead!!
The term "Pony" is a relatively new term anyway, and has changed twice in my lifetime (at one time Ponies were under 14 hands now they are under 14.2hh).
It also seems to cynical old me that the more like a squat little pony a papered Miniature Horse looks the more it's owner will try to bully you into believing it is a wonderful, distinct, breed of rare equine!!
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Oh and the US Mini and the European Mini are bred to exactly the same standards- the US Mini has it's footings in the same stock- to whit the thousands of mine ponies and the hundreds, possibly thousands of UK Shetlands that were exported to the US by the boatload when the Hype first started!!!
To my knowledge there is little or no truth to the myth that they were bred down form large horse- this does not work, I know I have tried!!!
They are NOT a breed they are a height registry.
A breed breeds true, Minis, unfortunately, do not, not yet.