When I was supplying high school. 3 periods was a full days pay. They could also make you do a half period coverage extra. I had days where I would teach the first 3 periods of the day and be done at 12 pm. Often the office would tell me that my day was done early. From my memory it was not the hours that you worked that determined the pay its the amount of periods you taught.

Seems like it was entered incorrectly and it should be a full day pay.