Estimates Are Based on Current Knowledge

Story Points represent reality based on current knowledge of the relative size of the work, as opposed to wishful thinking.

I've seen a number of situations where because the team has a rule that User Stories sized 13 and above do not fit into a Sprint, they arbitrarily size a Story to 13 because the Product Owner or Manager wants the work done in this Sprint. If the Team makes a commitment to the work it is expected that this work gets done. The Team decides how much work than can take on and are accountable to the commitment made. Forcing Teams to take on work they know they cannot do is counterproductive. The Product Owner or the Manager needs to prioritize the backlog so that the most important work gets done first. If the work is too big to complete in a Sprint it must be broken down to something that can be. The remaining work after splitting needs to be work by another team, for example.