> From a historiographical perspective, the gospels should probably be taken just as seriously as any other biography in their genre (e.g., Agricola by Tacitus)

Your example was written by the son in law of Julius Agricola, the gospels on the other hand.. Well, there are many theories of the authors there. Then there are the rewrites/reinterpretations... All with a lot more vested interest in religious motives over historical accuracy. It's been a while but I'm pretty sure that Julius Agricola's biography didn't include unfounded shit like miracles, which would further discredit the authenticity of the text. I'm also willing to bet there aren't even half as many mutually exclusive versions that again, discredit the authenticity of the text.

Tl;Dr - One is nonfiction, the other isn't.