Why am I getting nondeterministic results with the deterministic function STDEV()?
row_number is not deterministic if there can be ties (i.e. rows with the same PartitionField and DateField values).  Any of the tied values might end up with a PartitionRowId of 1 which would presumably change the final result.  

You could use rank instead of row_number but that would cause you to consider all the tied rows which may not be what you want.  You could also update your analytic function to add additional order by criteria to ensure that row_number returns a deterministic first row.