SQL Server Query returning all rows in a table instead of MAX()

Im trying to return the highest/last hour meter reading for each piece of equipment in an inventory, however, the query below lists all of the instances even using MAX.

select      sil.[Posting Date], 
            mre.[Service Item No_], 
            sil.[Job Code], 
            max(mre.[Reading]) as 'Hour Reading'     

from        [$meter reading entry]mre left outer join
            [$service invoice line]sil on mre.[Service Item No_] = sil.[Service Item No_]

where       sil.[Job Code] = 200


group by    mre.[Service Item No_], sil.[Job Code], mre.[Reading], sil.[Posting Date]