I honestly thought that Apple's market share in Japan was around 70 percent or something.
Good memory. When the iPhone 5S/5C first came out on a new Japanese carrier last October for free, it temporarily boosted its share up to 76% of total sales.
Variation from country to country or from market to market may be due to a number of factors, but one of the big ones is what the telecom carriers are promoting in any given quarter.
Yes, plus overall sales are almost directly correlated to how much the iPhone is subsidized.
The iPhone is "free" in Japan, and heavily subsidized in the US, GB, Australia, and thus does very well in those regions.