I always see SP a combination of time and risk. I think a lot of people do not include risk in the estimate.

So a story might be estimated at 3SP to implement but there's a high risk that it would blow out (e.g. idea was not fully proven in a PoC, work is in an area that is historically underestimated, reliance on a different team, etc.), so we set it to 5SP to include that risk. Maybe 50% of the time it does get finished in what a normal 3SP would finish in, but at least we've covered the 50% of time it blows out.