It will show "All" because in your query you are specifying a different member.

So in the context of [DimTagInstance].[Site Id].[Site Id] the calculation [DimTagInstance].[Latitude].CURRENTMEMBER.Name will return "All" because you are not including the [DimTagInstance].[Latitude] members in the query. If you did something like

WITH MEMBER

Measures.Latitude AS

[DimTagInstance].[Latitude].CURRENTMEMBER.Name

SELECT

{[Measures].[Count], Measures.Latitude} ON 0,

{[DimTagInstance].[Site Id].[Site Id]} ON 1

FROM

[Cube]

WHERE

[DimTagInstance].[Latitude].&[b]whatever[/b]

Then it would display the value of whatever.

CURRENTMEMBER is contextual to whatever query you are running so the fact that it displays "All" in your query is correct behaviour.

I hope that helps 🙂