Po-210 is an Alpha emitter, which does 20x more damage when inside your body than gamma. Co-60 is a gamma emitter. When assigning dose, and calculating limits, ONE of the things they consider is the type of radiation.
I haven't checked your math, but assuming you were correct, the Po-210 would give you a dose of 480 uSv/yr.
Additionally, they also take into account other limits, such as the Toxicity. The limit of Pu, for example, is based more heavily on the toxicity than the radioactivity.
Also, the shorter half live you mentioned, also means that MORE energy is emitted when it decays, which means it does more damage. The candle that burns brighter, burns hotter, and burns out faster.
So, Po-210 is more hazardous, and has a lower Bq limit, because of other hazards, not just the radiation aspect.
Po-210 is very poisonous (toxic). It's much less a hazard in a sealed source.