Which is fine. The point (heh) is that the mapping can be adjusted in a feedback loop based on what you actually did in the past.

I've always seen there being a maximum size of a ticket for one sprint. So effectively 8, or, 13, or 21, or whatever, is two weeks, and all the other sizes are some fraction of that, and you end up trying to calibrate how you vote for points based on how long things take.