فَإِذَا أُوجِبَ مِنْهُ شَيْءٌ بِإِلَّا جَاءَ الْقَصْرُ ضَرُورَةً
When one possibility is affirmed through illā from the general domain, restriction follows necessarily.
Negation initially ranges over the relevant class of possibilities. Illā lifts the negation for one specified person, manner, object, or time, while the remaining alternatives stay denied.
Thus “mā jāʾa illā Zayd” means more than “Zayd came”: the other relevant potential comers did not. Restriction emerges from the combined force of negation and exception.