Why do gospel contradictions matter?

[Jenny A]

Imagine you are a policeman attending the aftermath of a supposed armed robbery of a shop. If all of the cashiers claimed the masked man entered the building at precisely 20:47, would you be more or less suspicious about their story than if they had given slightly varying times? "10 to 9", "about quarter to 9", "just before closing time at 9"...??
The differences are natural and demonstrate that there wasn't a conspiracy to invent the entire story.

A more realistic analogy:

Imagine you are a policeman in New York taking a report of a supposed robbery of a store in Boston. The store owner who witnessed this robbery died ten years ago. He spoke only Korean. Four immigrants from Singapore who do not speak Korean and live in New York and L.A. want to report the robbery twenty years after it happened. None of them claim to be eye witnesses. One of them tells you the robbery happened on Christmas Day. Another tells you it happened on Christmas Eve Day.

They all say that though the robber was from Singapore, the dirty Koreans made him do it.

Oh, and by the way, when the robbery happened the sun stopped in the sky and and all the graves in Boston opened up and the bodies flew into the sky. Or not, depends on which guy you talk to.

Then they tell you the store owner was a saint who never sinned and was born of a virgin. He was born in New York because all immigrants had to return to their port of entry in 1980 for a countrywide census. But he grew up in Boston. His parents fled to Canada after the birth, or not, depends on who you talk to. One of them tells you that the store owner was born in the year Gerald Ford was President. Another tells you that Bush Sr. was then Vice President. (The fact that Ford wasn't president in 1980 and Bush was vice president then and Bush was never Ford's vice doesn't make them any less certain. The census never happened either. )

Though they have all this detail about the store owner's birth, none of them can remember what year the robbery took place.

Then a fifth guy from Korea shows up and tells that after the store owner died he came back from the dead to tell him about the robbery.