Casualties also include wounded and not able to return to the fight.
Zelenskyj just talked about the ukrainian dead soldiers, not wounded.
Russian losses but also just estimates based on engagements they had. I wonder how or even if Deserters and casualties caused by weather and sickness are counted. Also self mutilation is a thing in wartime.
These are russian casualties Ukraine has a hard time to measure.
But also casualties ukraine probably suffers from.