Power BI Filtering a column vs Filtering a table when is the difference apparent and when not

Your Case 1 can be written as:

EVALUATE
ADDCOLUMNS(
    VALUES( 'Product'[Brand] ),
    "FilterCol",
        CALCULATE(
            [Sales Amount],
            FILTER(
                ALL( Sales[Quantity] ), -- this only filters one column
                Sales[Quantity] > 1
            )
        ),
    "FilterTab",
        CALCULATE(
            [Sales Amount],
            FILTER(
                Sales, -- this filters all the table BEFORE context transition
                Sales[Quantity] > 1
            )
        )
)

Now, do you see the difference?

In Case 2 you get what you get because there is no context transition for the measures as was in the case of ADDCOLUMNS. The filters are already in place when the measures get down to work.