Complex sentence: For some employees, salary is paid on monthly rates, independent of how many hours the other hand, the hours of hourly employees can often be varied, so this type of labour cost is a variable cost.
Split into simpler sentences: For some employees, salary is paid on monthly rates, independent of how many hours the employees work. On the other hand, the hours of hourly employees can often be varied, so this type of labour cost is a variable cost.