User blog comment:Silver-Haired Seireitou/Sake Time with Sei - Sternritter and their Powers/@comment-3008090-20140327173621/@comment-1493741-20140327190839

His bloodline isn't an "alternate" as such, he's just from both mixed and pure heritage (Which still makes him mixed), which is apparently unheard of amongst Quincy history seeing as he's the only mixed Quincy known to have survived Auswählen. Its clearly a plot point in the arc itself, and is Yhwach's primary interest in Uryū. The specifics are likely to be explored (Unless Kubo does what Kubo does), but until then one could very well assume that being the offspring of a Gemischt and Echt Quincy puts a damper on Auswählen's effects.