Mirtazapine is an unusual drug, with a pharmacological profile very different to that of most antidepressants. Notably, it's a powerful hypnotic - it makes you sleep - and it increases appetite. Patients on mirtazapine in the present study put on over 2kg in 6 weeks.
Why does this matter? Because the two scales used to rate depression in this study, the Hamilton Scale
and the Montgomery-Asberg Scale
, both count reduced appetite and sleeplessness as symptoms of depression. If you're on mirtazapine, you're unlikely to have either problem - you'll be more worried about the exact opposite, insatiable hunger and drowsiness. So mirtazapine could reduce your total score on these scales even if it didn't change your mood.