The contractor user type is a bit different from the others, as it is aimed towards users who work irregularly.
You are charged the contractor fee for each day they are scheduled to work or enter time - so if neither happens for a certain day you are not billed. The price for doing both on a single day is the same as only doing one or the other.
The trigger for daily charges for the Contractor user type could be adding time or materials to a job or if the contractor has been scheduled for a custom event or job.
If the Contractor was scheduled for a custom event or job, then the daily charge will trigger on the day the event is set for.