I don't really understand why so many Christians are bothered if the story was 100% truth.

Let's analyze a different story: 300 Spartans.

Were there 300 Spartans, 500 or 1547 + 4393 others?
Were all the dialogs 100% realistic?
Was there a man with that exact name and surname who committed a treason?

Even if none of the above is true, the story is still powerful, universal and glorious. It's also inspiring and worth following.

Story about Christ = story about every extraordinary man ever, who wanted to change the world. Instead of accepting him, world crushes him (like Tesla for example) but his idea was so good that it transcended to eternity and indeed changed the world.