Back up - Activity report shows incremental back completed but differential requested

Hi there, I am a new User and have just set up my Back regime. In the settings I asked for daily differential back ups after the initial Full Back up. However I notice from the activity report of the Back ups completed that the Full Back up was successfully completed on day 1, on day 2 the activity report said that an incremental back up had successfully completed. Not the differential back up that I was expecting. Is there an error or have I done something wrong?