Не знаю, какая у вас сфера деятельности, но согласно ссылке ниже разница в уровне квалификации временных сотрудников:
temporary - низовые исполнители
interim - обычно руководящий уровень - менеджеры, директора и т.п., отсюда "временно и.о.", чтобы подчеркнуть их статус

См.:
Our experience of the market tells us that the terms “Interim”, “Contractor” and “Temporary Worker” tend to be used as a way of differentiating the perceived skill levels, creating a hierarchy amongst candidates who fill temporary positions.

Typically:
•An Interim Manager operates at a senior strategic or executive level within an organisation. Typically engaged to deliver a strategic piece of work or to cover a senior post until a permanent hire is placed.
•A Contractor brings a high level of skill and knowledge in their chosen field and is paid a day rate for their services. The majority of Contractors operate through a Limited company or use the services of an umbrella company.
•Temporary Worker generally operates at the doing level and may typically be paid hourly. Many temporary workers do not operate any sort of company and are paid through PAYE by their supplying agency

http://www.arraspeople.co.uk/camel-blog/projectmanagement/kn...