I'm not sure anyone other than the coders knows the exact answer to that.
Once upon a time, Openboxes had a super-fast blind scan, but tended to miss narrow transponders (lower SR than 3000 or so.) Then the blind scan was changed to find narrow transponders reliably, but it also got slower -- a LOT slower in some cases. Finally, a third blind scan appeared that got most of its speed back while retaining almost all of its accuracy. At this time was when the Accurate Scan option appeared, which scans at the rate of the previous slow blind scan, but doesn't find anything extra 99.9% of the time. So nobody is sure exactly why it's there. Maybe it would be a little better on weak signals (although most of the time it doesn't seem so to me), but the Blind Scan seems fine almost all of the time on narrow transponders now.