The range of a surface to air missile is generally an effective range where the missile will likely hit the target as long as the target stays within the velocity, vector, altitude, maneuver envelope at that distance.
The missile's effective range will be approximately where it has exhausted its fuel and attained its maximum altitude. But after this point the missile will likely traverse a distance close to its effective range and may detect and home on a target on its way back down.
Somebody may have been short calculating the size of the danger area.