Rankx Quirks

Hi User5231 
I think it is the context transition. Context transition does not apply to variables as they are evaluated once. This is why rank2 does not work.
I agreed with you on rank5 but if you think about it when refering

[Sum by Category Filter Method]

it is actually translated to 

CALCULATE (
    CALCULATE (
        SUM ( 'Table'[Time] ),
        FILTER (
            ALLSELECTED ( 'Table' ),
            'Table'[Sub Category] = MAX ( 'Table'[Sub Category] )
        )
    )
)

 As the engine wraps any measure referenced in any kind of calculation with CALCULATE. 

Just a thought. you may try and confirm.