Update Query Gives Different Results Each Time Run - SQL Server 2000

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SQL Server 2000 - When running the following update query I get a different set of results each time the stored procedure is run. I ran the stored procedure using a job 4 times, got 4 different results.
If I run the update query directly in Query Analyzer, my results are always the same.
The query:

    update COrdHz
	set Racf = a.N_server_id
	from nbtord a
	where a.ord_nbr = COrdHz.order_num
		and a.ord_supp = (select max(ord_supp) from nbtord b 
	where b.ord_nbr = a.ord_nbr and rtrim(ltrim(n_server_id)) is not null)
		and ltrim(rtrim(N_server_id)) is not null

Has anyone experienced this inconsistency?