Thanks for the explanation, I see exactly what you mean. When going to the Japanese website both are grouped when talking about the details of the driver. Although I see AD2000X in parenthesis for a few pictures - have no idea what they're talking about. Also on the US amazon website, when talking about about 2kx driver it says "designed exclusively for the ATH-AD2000X" and the 1kx drivers "The 53 mm large-aperture drivers are exclusively crafted for this model". I don't really trust the product description from Amazon as they're typically misleading or wrong, especially for imported products.
Amazon doesn't write the descriptions of products shown for sale, sellers do. One item might have four listings with various descriptions of the same product because four different sellers (manufacturer or not) wrote the wording for each page.
Important to do your own research from manufacture's websites and official review sites or from trusted reviewers.