Maybe my assumption is wrong, but I believe Report Tags are just a “friendly” way to create SQL, which under the hood is what I believe is happening. If that is the case, the result is correct, because it is counting Order lines, rather than the Quantity field that is part of the same table [Orders].
I just did a good example of this (in SQL) on another Topic. In the first query we’re counting tickets using COUNT(), but he wanted counts of Burgers sold, so we had to query the [Orders] table, but not use COUNT() which counts records; instead use SUM() of the [Quantity] field.