How does Burndown report query for iterations when the current iteration checkbox is selected as a report parameter?

First, the report queries for current iteration and gets a list of iteration names (not ID) from different timelines within the project. It then looks for the current iteration of the selected timeline from the list of iterations returned by the first query.

For example, a project has the following timelines and iterations:
timeline 1
|_ sprint 1 (current)
|_ sprint 2
timeline 2
|_ sprint 1
|_ sprint 9 (current)

Run Burndown report. Select timeline 2 and the 'current iteration' checkbox as the report parameters.

The report engine asks for all current iterations in the project area. Note that only iteration names are returned:
Result ==> sprint 1 and sprint 9

Then, from the above result, it filters for iterations that belong to the selected timeline:
Condition ==> timeline equals 'timeline 2' AND iteration in ('sprint 1', 'sprint 9')
Result ==> sprint 1 and sprint 9

Since sprint 1 also exists in timeline 2, it is returned by the above query. The report then shows data from both sprint 1 and sprint 9 of timeline 2.