I'm not sure about the exact difference, but here's my guess. Rome can only build her own legionary units and mic's in her government 1-settlements. The difference between a lvl2 and a lvl3 would be both economic (more civil and economic buildings in 2) and military (more local forces in a lvl3 than in lvl2). Not that the romanoi can train many local units on the steppes, but it might make a difference. Given they are capable to conquer the steppes and deal with a nice distance-to-capital-penalty in the first place.