Not looking at auto-metamagic spells; metamagic spells require X spellslots HIGHER than normal, meaning altering them makes them more complicated. Because of this I've always assumed that a hastened spell is actually a more complex but simplified variant of the normal spell.
It could also be seen as a way of just triggering the last few bits of the spell in order to complete it. Similar to a scroll already having all the incantations, materials and components done at it's creation, only needing the remaining triggers in order to complete and cast it.
Found something somewhat related
here.