There's a shift of command in the house of Xperia. The Sony Xperia X and its two siblings are taking over from the Xperia Z. A dynasty has come to an end. We hear the C and M families are being put out to pasture as well, soon it will be all X top to bottom.
Source: http://www.gsmarena.com/sony_xperia_x-review-1441.php