In a word: Marketing.
Mongolian hair is marketed as more durable than Chinese and lighter than Indian.
The term “REMY” refers to the characteristics of the hair and the method used to manufacture the hair extensions Basically it all needs to be unilateral (going in the same direction) or it will start to get tangled, frizzy and start shedding pretty quick. . Most commonly it's Indian but can also be Asian or European. There is a big push in Russian and south east Asia hair at the moment.
Mongolian hair like any other extension can be Remy.
You can name almost any country and there will be claims of the shiniest softest most natural, virgin hair.
Transform do Mongolian, their based in Swansea too so you could go to her and see for yourself before buying, I currently have both remy & Mongolian from transform in my hair right now, their a different texture to each other, the Mongolian is definatley going to last a very very long time it's beautiful