Even if half of iPhone X buyers have bought the larger storage model, it's hard to make the ASP number work with iPhone X only accounting for 20% of iPhone units. We'd have to, among other things, believe that Apple sells very few low price models (e.g. the iPhone SE) in emerging markets. And if it's true that Apple sells very few low price models, then the even distribution model (which trip1ex described) which gets us down to 14% for iPhone X share wouldn't work. (And, of course, 20% is more than 14%.)
When we refer to ASPs, we aren't talking about average retail price. We're referring to the average that Apple gets, with certain adjustments (e.g. deferrals for future software updates and from prior sales). Those numbers are also affected by, e.g., channel builds and draw downs. We saw the latter this past quarter, and most likely the iPhone X accounted for a disproportionate share of that.