It's because of the scan logic of the radar.
In SAM the radar will visit the bugged target at intervals but won't center the elevation on said target, and further more the elevation is now locked (which I think it shouldn't be) to whatever it was when you transitioned to SAM. In TWS the radar will center both elevation and azimuth on your bugged target, hence the bandit will have a harder time escaping.
Someone said somewhere on here that SAM features were not finished yet.