I have noticed that your burndown calculation is a forecast one - this new calculation mechanism was introduced as a replacement of the previously used trend mechanism to improve the handling of increasing the remaining work by adding new work items. The difference between the trend and the forecast mechanism if that the forecast mechanism doesn't predict the addition of new work based on the work added before and calculates the forecast based on the work remaining at the moment and the current trend for completing tasks.
As breaking the stories into tasks, usually done during the first few days of the iteration, often leads to adding new work and setting more precise estimations, the about even or sometimes even upwards slope of the actual line in the burndown is normal and the forecast will be downwards because the prediction doesn't follow the upwards slope from the new work added.
The Iteration planning screen used by default estimates for the capacity calculations presented on the iteration cards. However, the burndown mode used in the Iteration status screen and in the reports is the default mode selected in the project settings.
Do not hesitate to get back to us if you need any further assistance.