Ion implantation is probabilistic, in where the ions end up,
and "hard mask" features are able to affect this too.
Halo implants tend to be shot at oblique angles from
normal so they can "reach under" spacer. If gates are
too tightly packed, it becomes sort of like getting sunshine
on the sidewalk, downtown - some places are better
than others, depending on the neighbors.
A local weakness in the halo implant will raise that device's
leakage. If too many ions bounced off (backscatter) or
were caught before target depth (shadowing) the halo
shot will be weakened.
Halo works by stiffening the body doping and pushing
harder into the LDD at higher drain-body voltages (a
JFET action).