why were the couple left astonished after thr burglary

Which chapter are you referring to?

Mr and Mrs Bunting were astonished because they heard , felt and even seen the presence of a person in the house and his actions but were unable to catch the thief / burglar that broke into their house . They were even sure that the burglar was a person from the village but they couldn't see who entered their house , took the money from their reserve of money, who lighted the candle, who opened the kitchen door and slammed it and went out.