THE Y-DNA haplogroup ONLY provides information about one line - your father, his father, his father and so on. No grandmothers and no other line. The simplest explanation is sometimes the correct one. At some point on your father line there is what is called an NPE (non parental event). It can be an adoption, an illegitimate child, a child fathered by a man not the recorded husband of the mother... You get the picture. Since the haplogroup goes back for thousands of years, it can have happened at any time from one generations ago to one hundred.