Guided Project Answering Business Questions Using SQL - confusion on sales by country (screen 5)

Screen Link:

My Code:

The solution query

Attempted query using invoice total

What I expected to happen:

unit price solution results

What actually happened:

invoice total results


Comparing to the solution query I do not understand why unit_price in invoice_line table is being used to calculate total sales instead of total in invoice table. When I calculate totals by country I get the same total, however I get different results when trying to use SUM(total) in context of the larger query. My logic is that unit price is dependent on quantity which happens to be 1 for this data set, however the totals calculate to the same amount and would require one less join. Any assistance would be greatly appreciated. This is my first post after not being able to fully conceptualize this issue. Thanks for the consideration.