Burndown Estimated Completion Dates

I have talked to many of the experts and consultants on this but no one can seem to give me a straight answer on how the estimated completion date is calculated on a burndown chart. The help article says it is based on the "rate of points completed". I am not entirely sure what they mean by "rate". I have also been told it takes into account if points were added throughout an iteration and resource allocations as well. What I am looking for is the exact formula or algorithm it uses to justify the completion date. This is a bit hard to show in text, but here is what I am seeing. I'll try to keep it simple.

iteration duration = 10 days

iteration capacity = 10 pts with 100% focus

iteration estimated points = 10

Lets' say we complete 1 point a day for 5 days. That would mean we have 5 days left and we are completing 1 point per day. To me, the estimated completion date should be the 10th day because we are on pace to finish by then. Instead, the est comp date will show +3 working days. Can anyone explain why we would be behind 3 days if we are completing points at a rate consistent with the duration and estimated points for that iteration?

Any insight would be greatly appreciated. Thanks!