sql server - Inconsistent SUM when using OVER PARTITION in MSSQL -

on static table, i'm trying run following query in ssms (to latest transaction each user , sum dollar value):

select     sum(nmoney) totalmoney (     select row_number() on (partition ngroup, nuser order dtransaction desc) seqnum     , nmoney     [mydb].[dbo].[mytable] ) mysubquery mysubquery.seqnum=1 

this table 2,701,510 rows , nmoney column of type decimal(12,2). when run multiple times, varying result:

2317367341.75 2317370443.45 2317449819.62 2317360649.43 2317449819.62 

what causing inconsistent result?

for floating point arithmetic order numbers added in can affect result.

but in case using decimal(12,2) precise.

the issue duplicate values ngroup, nuser, dtransaction row_number not deterministic different runs can return different results.

to deterministic behaviour can add guaranteed unique column(s) end of order by act tie breaker.