Month() should return a dual value (check out the Help for more details on Month and dual value).
So it carries as well a numeric as a text value. You can force QV to return either one by using
If you don't force QV, it will return the value depending on the context, i.e. if comparing to a numeric value, it will return the numeric representation. If you use Month(DATEFIELD) as dimension, it will probably use the text value.
Where do you use your dual (i.e. [Weekly Month] field)?