They were written within the lifetime of the apostles and well they had pretty good memories back then so what they wrote down is pretty accurate.
Oral Tradition my friend. Oral Tradition. The scribes back in the day had to commit entire scrolls of scripture to memory and also the teachings of their Rabbis. That's was their M.O. Also, most people were illiterate so people passed info along Orally and it was a communal preservation. In other words, ten people witness an accident (for instance) and those ten people recount what they've seen from their perspectives (maybe one man was standing where he could see things the others did not and then as those in the community listened they would commit what they could to memory and retell the story in the company of the community wherein if anyone strayed from what was in the original reports, the community member that noticed the error would correct them.
Back in the day anything written down was pretty much considered suspicious because it was seen not unlike we view rumors in our culture because we have the opposite mind. We view what is written as being more valid than whatever gets pumped out the rumor mill.
Consider giving false testimony and the consequences (legally) of doing such a thing and also any social repercussions that come with lying or adding something that didn't have a valid source (like a person that actually witnessed the event being retold). If we had those standards today our journalists would be stoned. lol
Yoinks
Oh, and added to edit: There was talk at one time to change the four gospels into one gospel in order to erase the variances but they left well enough alone and knew that despite the "conflicting" reports that might sway a scientific eye that most of these stories were being told through a theological lens and the theology is solid.