Story Points (SP) is a good measurement, that allows estimating User Stories.
SP (in my understanding, it's an important note) is convenient because it is relative and not related with time. All estimates are based on comparison with a common basis. But what if two stories have completely different bases?
For simplicity, let's look for an example that is not related to any specific area. Like digging a pit.
Let's assume, that it will take 1 SP for digging a pit one-meter depth. Accordingly, we need 2 SP for making a pit two meters in depth. Very convenient. We don't care about our equipment. If we have a shovel then 1 SP will map to 1 man-day. If we have an excavator then 1 SP will map to 1 man-hour. But two meters pit anyway will take to us twice more time then one-meter pit.
All is good with pit digging. But what if the next story is "catching of an elephant"?
Should we make estimation with another (elephant related) basis? In that case, we will have two kinds of Story Points, which will be absolutely different.
Or, if we know, that 1) catching of one elephant will take for us 1 man-day 2) we still use a shovel for digging, then we should estimate this "elephant" story to 1 SP? This suggestion sounds absolutely wrong to me. But I wrote it because in my practice people use this option in most cases.
Or, maybe, I just misunderstand the concept of Story Points?