I have two queries:

1.

SELECT 'Total', sum("Units sold grouped by country"."UnitSales") FROM  "Units sold grouped by country"

Result: Total | 1051

2.

SELECT 'Total', sum("Top 10 Countries"."UnitSales") FROM "Top 10 Countries"

Result: Total | 806

So, you can imagine what I'd expect if I tried to merge the two into a single query:

SELECT 'Totals', sum("Units sold grouped by country"."UnitSales"), sum("Top 10 Countries"."UnitSales") FROM "Units sold grouped by country", "Top 10 Countries"

Result: Totals | 10510 | 37076

These numbers do not match expectations, as shown when you do them separately. I suggest that something strange is happening.