Because the gospels disagree with each other on several things (the tomb, the last supper, etc.), Christians, including JWs, are forced to to an interesting thing. Not only do they declare each gospel right AND wrong, they synthesize them to create a 4th narrative not found in any of the books!
If we see people doing that today, why are we surpised at all it happened back then? Basically, making it up as you go...