CDO

Hi, I am seeing different results when I use ymonsub and monsub on a file which has only a single year. Am I right to expect that they should produce exactly the same answers?

I've attached the file, which is a function of time, level, lat, lon. The following commands produce different answers:

cdo -monsub test_file.nc -monmean test_file.nc tmp.nc
cdo -ymonsub test_file.nc -ymonmean test_file.nc tmp2.nc
cdo infov -timmax -vertmax -fldmax -sub tmp.nc tmp2.nc

1 :       Date  Time    Varname      Level    Size    Miss :     Minimum        Mean     Maximum
1 : 1958-12-31 00:00:00 vwnd             0       1       0 :                  12.040

Interestingly, when I extract just a single level or a single box, the difference is zero:
cdo -monsub -sellevidx,6 test_file.nc -monmean -sellevidx,6 test_file.nc tmp.nc
cdo -ymonsub -sellevidx,6 test_file.nc -ymonmean -sellevidx,6 test_file.nc tmp2.nc
cdo infov -timmax -vertmax -fldmax -sub tmp.nc tmp2.nc

1 :       Date  Time    Varname      Level    Size    Miss :     Minimum        Mean     Maximum
1 : 1958-12-31 00:00:00 vwnd           500       1       0 :                  0.0000

Am I doing something incorrectly, or perhaps these functions don't support using time, level, lat, lon data?

Any advice would be much appreciated!