The voting pool manager can accept an arbitrary number of public keys from Sybil, who may vote as many times as she chooses.
In a paper voting system, Sybil is limited to how many polling stations should could visit in a day, because her physical presence in the station would be noticed by station watchers if she attempted to vote more than once at any given station. Even then, there are additional checks and balances, because she first needs to register herself at each polling station.
In a purely electronic system, nobody is able to observe Sybil.