Fractional transfer methods are designed so that the result of the election remains the same when the order of the ballots is changed. The basic idea is that, when transferring a candidate's surplus votes, all of the ballots are transferred but at a fractional value. The fraction is set so that the total value of all the transferred ballots equals the candidate's surplus.
With fractional transfers of votes, secondary surpluses must be allowed. Since the point of fractional transfers is to ensure that the method is independent of the order of the ballots, all the ballots transferred in a given round must be treated identically.
Since floating-point arithmetic, some implementations round all calculations to a number of decimal places. This implementation does all computations in floating point.