It was my impression that this was simply so because the soviet faction is designed for poor performance in general.
However, a number of suggestions have been made in this thread and ofc. they are worth trying. It's just funny how those hints are always followed by 'this might not always work because...'
unlike COH1 latest patch, in COH2 grenaders can miss, doing pretty much no damage, even if visually you see them hitting the enemy. It depends from the distance at which you throw the grenade, you can see this more times on rifle grenades of Grenadiers when they shoot from max distance, but it happens also to Guards and Shock Troops grenades.
I never noticed any difference for the grenadier nades. Seems like they cut out about 2-3 men regardless of distance. And speaking of nades into buildings, the gren nades seem much more effective.