How does a dispatcher assign relative costs to trajectories in TOS?
[The] TOS options include defining preferences with regard to:
  
  
  Route: The Flight Operator may route out of a CTOP FCA into another CTOP FCA or out of the CTOP altogether. Submissions should conform to standard flight plan requirements (i.e. NRP, required reroutes, preferred routes, etc.).
  Altitude: The Flight Operator may file different altitudes, thereby changing the assigned TOS option.
  Speed: The Flight Operator may file different speeds, thereby changing the assigned TOS option.
  [Relative Trajectory Cost]: The Flight Operator may file different RTCs, thereby changing the assigned TOS option.
  TVST/TVET: The Flight Operator may file different TVSTs/TVETs (start/end times), thereby changing the assigned TOS option.
  RMNT: The Flight Operator may file different RMNTs, thereby changing the assigned TOS option.
  


Source: AC No: 90-115





For example, if they change only the route, the distance will change, and so would the average winds, and thus a new air distance estimate. It could be longer or shorter. The airline may also change the speed, flight level, etc.

Calculations are most certainly computerized but with human input/decision. It could be made either by using proprietary software or commercial flight planning solutions.