More interestingly is the fact that no-one seems to notice that Rita Hayworth has been exchanged for Raquel Welch.
The original novella is entitled Rita Hayworth - The Shawshank Redemption - probably because the inference is that the novella is set at an earlier time than the movie.
These changes and questions like the poster sticking are all part of what makes films what they are - we have to suspend our disbelief, as with the thunder coincidence, and just go with the atmosphere of the film.
Otherwise we'd be sitting there wondering how Red (who is called Red because he is of Irish extraction, and has red hair) is being played by Morgan Freeman.
I can gloss over those sorts of things, no problem.
When it comes to Sean Connery playing an Irish cop in The Untouchables with a Scots accent, and then getting an Oscar for it (????? !!!!!) I do draw the line, but Shawshank is a fine film, and the intricacies of basic physics do nothing to detract from its appeal.