Because of the long time between the two events, there would be possibility of error in exact words and perception of its meaning.
Explanation:
After the twenty long years Douglass might not have a clear picture of what Lincoln had said. There is a high possibility that after this much long time he may have subconsciously or unconsciously modified what Lincoln had said a little to be what he wanted to hear. It is really difficult to recall the same words or meaning from an event, after twenty years of the event actually occurred.
