Ranking - problem when filtering only one value

Hi All,

I have a problem with RANKX function. I need to prepare a report in which a user will be able to select one category value, and will see rank in terms of a measure of that value.

I've prepared a DAX function, but it doesn't work in each case, sometimes the rank of selected category is different than the actual (usually by 1 place).

My data table looks like this (40k rows):

I want to rank values from the 'Category A' column by the sum of values in the "Measure 1' column

Dax function: 

Rank Category A by Measure 1 = 
IF(
    HASONEVALUE(data[Category A]),
    RANKX(
        ALL(data[Category A]),
        calculate(SUM(data[ Measure 1 ]))
    )
)

It works fine for most cases, but sometimes the rank number is wrong, as in the below screenshot. If we look at the bigger table with all categories then selected '6_Cat' has rank number 6 (and it's correct), but when we filter out only 6_Cat (table on the right) then its place is 5 (what is wrong).

I've noticed that when there is less data then the wrong rank appears less often, but still, it is.

Would be very grateful for the help, thanks in advance!