stilton, and their-in lies the difference between a MEZ and akill-zone.
To use an old set of data for the SA2. The MEZ was 40 but the kill zone was 27. In order to get a kill at 27 miles the missile had to be launched when the target entered the MEZ at 40.
If the target was non-cooperative, ie it evaded, it would remain clear of the kill zone until the missile ran out of ommph.
In the Learjet case I would opine the Learjet flew in to the kill zone in ignorance whereas the 707 was engaged when it entered the MEZ but then avoided the kill zone when it detected the missile launch.