Why is Barker untrustworthy in Act One?

Barker is untrustworthy because his view of the events does not seem possible.

“Well, then, everyone is agreed that after the alarm was given only a minute at the most had passed before the whole household—not Mr. Cecil Barker alone, though he claims to have been the first, but Ames and all of them were on the spot. Do you tell me that in that time the guilty person managed to make footmarks in the corner, open the window, mark the sill with blood, take the wedding ring off the dead man’s finger, and all the rest of it? It’s impossible!”