BS-RAC is the gained credit of the past x days divided by x, where x is the number of days in the day-history (also used for the 60 day history chart), with a maximum of 60 days.
Some projects to not correctly update project-RAC which makes this RAC too high or too low compared to reality.
BS-RAC is more accurate than project RAC because it is updated every day together with the daily update and it represents your true average credit per day.