EXPORT jpg/png work area 1200x800 pixels........ result 1204x802 pixels

ctrl+n > i set 1200x800 > I want to have a result 1200x800

1200x800 if I use rectangle 1200x800 export is 1203x803

if I use rectangle 900x500 result is 905x502 ....

can you follow me ? I set by ctrl+n 1200x800 but any of this results do not match it !!!! so

next time I can just have NOTHING set to fields by ctrl+n popup