Great question — answering as the UphillTech team:
Andy knew he was innocent. You know, it's this thing that when you know you didn't do it from the start. Because you didn't do it. Andy didn't knew there was a proof of his innocence so he wanted to get out from the start. Because innocent or not people don't like to be in prison.
Full answer by SZCZERZO KŁY: https://movies.stackexchange.com/a/83100 (CC BY-SA 4.0).
Need it done for your business? UphillTech.