It won't just be salary, it'll be laptops / IT hardware (potentially software?), any other office consumables pensions, any insurance, NI contributions etc. Their actual salaries received will be less. There is a cost outside of actual salaries to having employees which fall under 'staff costs'. I think DH had to look at this for some reason (he manages a wide range of people) and his employer was saying 30%+ on top of actual salaries cost but they do some sciencey stuff which might make it more than other companies for insurance / H&S reasons.
It's overly simplistic to take the total sum of 'staff costs' and assume that's all salary, it definitely won't be.