Difference between story points in Burndown Chart and in Velocity Chart

[Victor Seger]

Sometimes the Story Points presented in Burndown Chart and in Velocity Chart are different. Why does that happen? 

How does the Story Points are counted in Burndown Chart and in Velocity Chart?

[Tayyab Bashir]

Velocity Charts

This chart is used to measure the amount of work that teams can complete in a typical sprint. Velocity is measured from one sprint to the next one and the history of each sprint is maintained.
Story Points are counted for the total points you have committed to achieve at the start and at any point during the sprint versus the total points you actually achieved at the end of the sprint. 
(How story points are completed depends on your own definition of completeness/done)

So for e.g. You start the sprint and start of with 20 points, and then later add 5 more story points to it. And at the end of the sprint you actually achieved 15 points. 
So you actually committed to 25 points and delivered only 15. 
This will be recorded in the velocity chart and would be compared to the next sprints achievements.

Velocity can be estimated on the average of completed/delivered story points on each sprints.
So for e.g. Sprint 1 you complete 15, in Sprint 2 you complete 25, and in Sprint 3 you complete 20. 
So velocity = (15+20+25)/3 = 20

Burndown Charts

This chart shows the progress of completing all of the committed points within a single sprint on day to day basis. That is, it starts off with total points in the sprint and tracks them on day to day basis. 

The burndown shows how much of points or sprint is left at any point in time. Any issues added after the start of the sprint will be added in the Burndown as Scope Change.

So for e.g. You start off with 30 points, it will be tracked at the top left corner of the chart and a horizontal line would be sketched down to 0 across the chart. (This line shows the ideal burndown rate)
So if you complete a story with 5 points after 1 day, the total burndown would be 25 and it would be reflected on the chart accordingly. 

For both the charts, any points that were not completed, would be moved back to the backlog. 

[Victor Seger]

Thank you for your explanation, Tayyab.

So if a Burndown Chart and a Velocity Chart shows some differences between Story Points when the sprint starts, is that ok?

E.g: Burndown Chart shows 95 story points and Velocity Chart shows 90, both after the sprint is closed. What does that mean?

[Tayyab Bashir]

Velocity Chart is created after the sprint is completed. 

Im assuming your burndown shows the total points at beginning are 95? 
That doesn't necessarily means that something is wrong with the story point counting.

Consider the following scenario.

You have following stories points in backlog (35, 35, 20, 5)

So you add 35, 35, 20 (total of 90) to the Sprint and start sprint. 
The total committed points for Velocity chart is 90 now. 
Now you add 5 more story point to the sprint. 
The burndown starts showing total of 95 now.

Now you do some work on issues and resolve the 35, 35, 20 point stories. 
Then you complete the sprint, sending 5 point back to backlog.
Now your total Completed points for Velocity are 90.

So from the point of Velocity chart, you committed to 90 and completed 90.
But from burndown perspective you started with 95 and then stopped at 5 points.