>>just for testing, output the variable in each detail row of the report and watch the change. Also, check when does the variable reset.
>
>I don't know how to do either. That is, I don't know how to "output the variable in each detail.." And I don't know how to "check when the variable is reset."

You are kidding, aren't you? Sorry, but I can't resist.
1. Open the report (double click the report on the Project Manager)
2. Open the menu item Report / Variables ... by clicking it
3. In the left box, select the variable you like to check for reset with a single click
4. On the right hand, there is a Reset value based on. If the condition mentioned directly below is reached (for example, change of page), the variable selected on the left hand will be reset to Initial value
5. In the left box, open the variable you like to output in Detail Band with a double click
6. On the keyboard, press <CTRL>+C, <ESC>, <ESC>
7. On the Report Controls toolbar, click a "Field
8. In the Report Designer window, click into the area below the upper Detail *. If possible, on a free, white area.
9. On the keyboard, press <CTRL>+V, <Enter>
10. Repeat 6 and 7 on the lower Detail *
11. Run the report the usual way
12. In the preview / paper printed / PDF / whatever is the result of 9, check near the position you placed the field on 7, 9 there should be the value of the variable visible.