Factor investing is a way of earning premiums by taking exposure to factors. Robeco wants not only to document certain statistical patterns, but also to find the underlying explanation for these factors. The theory of behavioral finance also provides explanations for factors.
Premiums are often explained as representing compensation for risk, and are referred to as risk premiums. Research - some of which conducted by Robeco - has shown that premiums are not always the result of risk-taking (see explanation of unrewarded risks) and it is therefore better to use the term 'factor premiums'.