You commit the details of your high school graduation to memory, but later find out that your memory is not fully accurate concerning this event. this is primarily due to

The fact that memory is a constructive process.

Explanation:

Memory is a "constructive process." What this means is that memory is an ongoing process that is never over. Although you might commit the details of your high school graduation to memory, you might revisit them several times over time. This will lead you to add, modify or alter some of the information that you had previously stored. As the mind continues to construct the memory, this will continue to change slightly.