Figleaf, I think the story is, the demand for them is very high and supply is limited. They are definately uncommon to start out with.
Weepio, I did think the 1893 (502) coin was 1892 (501) to start out with. But the area where the "one" stroke is, is aligned to the bottom of the "hundred" character, instead of the middle of the "hundred" character. Finally, 1892 coins only come with the variety of "Great Korea" (大朝鮮 3 characters) versus 1893's only variety of "Korea". (朝鮮 2 characters)
Like I mentioned, I found this particular coin very difficult to replace. JNDA and the Korean catalog both seem to suggest that this coin should be easy to obtain but this is the best I can do for now.