You might want to check you default calendar settings to make sure you are working standard hours (8 AM to 5 PM). I would also suggest checking your project options to toggle the task type between fixed work and duration to see if that makes a difference. If neither of those fix the issue, then I would suggest looking at the details of each of the two tasks to see if the settings are the task level are different (ie one is effort driven, one uses a different calendar, etc.).
