human memory is not perfect. not 2000 years ago, not now, probably never. a story - especially one as long and complicated as an entire gospel - passed on by word of mouth cannot be assumed accurate. in generations of passing on a story by word of mouth, it's pretty reasonable to assume that some details were lost, changed, or embellished.

writing it down, copying it, distributing it, etc, could be more accurate, but then the person doing the copying might make mistakes as well. or they could modify it consciously. i'd like to see one biblical scholar who will say that the bible is 100% accurate history. i'd also like to see one historian contend that ancient word of mouth can be taken as 100% accurate history, too.

and, as others have mentioned, the bible as we know it didn't begin as a cohesive literary unit. there was a time when there were multiple versions of each of the gospels, and even more gospels than the moden bible contains. how can we know for certain the ones that made it into the modern-day bible are the right ones?