I mean, if a person is sensitive to insulin and only requires a small dose, then there is only a small amount of
insulin working, and if a goof is made, it may not affect
glucose levels very much.
One unit of insulin too much or too little is probably not going to make sugar levels spike too high or too low, or at least not dangerously so. If a person takes larger doses of insulin, then there is more margin for error just because more insulin is involved.