Because it cannot distinguish between either one. It says to seek god. Well, which god? The God of Christianity, or the God of Islam. Or are the two one and the same?
You have relatively few choices in that. Begin with the premise that the two books are diametrically opposed (which they are, but since you are unfamiliar, you must take my word, for the sake of the argument).
Given that you have examined the books sufficiently to give them the benefit of the doubt that they might indeed represent god(s), logic would have it, if you have winnowed the field down till there are no others, considering the two books are diametrically opposed, that:
1. They do not represent the same god at all, and there are two gods to choose from.
2. they represent the same god, but then one or the other book is not true, and you have two books to choose from.
3 neither represents god, and you must keep seeking.