The usual reason in Real Life is that -- and I saw a Venn diagram of this years ago ... someone tried to explain it to me, why another man's cousin was not also a cousin to his 1/2 brother ...
Jack was married to Joan (3 ch) and then
Jack married Jill (2 ch);
Jill was married to Harry (4 ch) and then
Jill married Jack (2).
Tried to figure out which kids are half-sibs. Okay, Jack & Jill had 2 kids, who are 1/2 sibs of Joan's 3 ch and Harry's 4 ch. But Joan's 3 ch are not half-sibs of Harry's 4 ch.
Now, in WT, it is as said by the others, usually one parent has not been credited with the child, even though they are parent and child and that is is fixable