Earlier quoted context omitted.
> story points = number of days to implement Some variant of this has been the case in every agile team I've ever worked on.
No one's ever been able to explain story points to me without saying something like "Story points are kind of like how long it will take to implement, except it's not that". So what is it then? All the explanations and examples are in units of time, but with a disclaimer saying that the true nature of story points is not time-based, except for the fact that they can only be explained in terms of time.
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.