There is setting under preferences for tracking payroll in minutes or decimals..i think this might be throwing off your math.... it will convert the number for you based on which you choose. 

Go to <Edit<Preferences<General<Time format   ...... if you choose minutes, you can type in 4.5 and it will convert it to 4:30  and vice versa if you choose decimals. 

4.2  converts 4 hours and 1 minute

4.5 hours converts to 4 hours and 30 minutes

pull  minute conversion chart from online, it will help you check yourself as you figure this out.