I had a look at the application on my computer and this is what I found regarding your first question.
The selection object on the top is making selections in the price field values. In your example you are only selecting transactions containing prices between 330.08 to 478.73. In my data model I have more than one transaction per customer and the sum of price could become larger than 1000. See attached screenshot below.
Reading your last comment it sounds like you want to make selection on customer based of sum of price . To do that you will need to adjust the distribution graph putting customer as dimension and sum(Price) as measure. See attached qvf.