Easy... Human failure in accurate recollection and the elapse of time between the first and the second statement, giving Shelley the time to gather his thoughts.
But the elapse of time favors the reliability of the earlier memory. To argue otherwise is to resort to the kind of special pleading one more usually associates with the Warren Gullibles we both despise.
How could Mr Shelley possibly have had a false memory, as of ca. 2pm on 11/22, of running out to the corner of the park and running into the woman at whose wedding he had only recently been best man? Absurd!
The corner of the park and the little old island across the street could easily be the same thing described differently on two separate occassions.
They are the same thing, only here they feature in two incompatible stories.
Again, I'm still not sure where you are going with this, but as far as I am concerned it's not a significant matter one way or the other.
It may be highly significant that Mr Shelley changed his story of his immediate post-shooting movements