Their function may overlap but they are really looking for different things. MWB is looking for general malware of all sorts, SAS is specifically looking for nosey unrequested tracking of the PC user, mainly cookies that hold that info.
Should say SAS has been acting strange for me too of late, except in my case it is finding hundreds of cookies instead of a few. Unsure why. Suggests either it is failing to remove them, if they exist, or something is rapidly putting them back ?!?