Thank you for posting about the hourly cost and payroll expenses.
The difference between them is the hourly cost is based on the Cost rate (/hr) while the payroll expenses are based on the Billable (/hr) and may include a markup. These two can be manually entered or modified when adding time entries into a project.
Hope that this clears up the difference.
If you have other questions in mind, feel free to go back to this thread.