Need Help on DAX - SWITH / IF Function results incorrect

Comparing two measures [Ref CY], [Ref PY] and deriving a custom tag for each record.

The calculation below works well for some scenarios but incorrectly calculates if it meets "ELSE" condition records. Any suggestion would be much appreciated.

Priority =

    IF

    (   AND

        ( [Ref CY]=0,[Ref PY]=0),"C",

        IF

        ( AND ([Ref CY]=0,[Ref PY]>=1),"b",

        "B")

    )

Priority Alt =

SWITCH

(

    TRUE,           

    (([Ref CY]=0) && ([Ref PY]=0)), "C",

    (([Ref CY]=0) && ([Ref PY]>=1)), "b",   

    "B"

)

The calculation works well

But it is incorrect if any record meets on ELSE condition