Why Does Cut Avocado Turn Brown Even in an Airtight Container?

I put half an avocado in an airtight container in the refrigerator, but the exposed surface still turned brown by the next day. Is the discoloration mainly caused by air left inside the container, or can it happen even when the fruit is covered well?

I’m also wondering whether browning means the avocado has spoiled, or if it is mostly a surface change. What storage methods actually make a noticeable difference for a cut avocado?