So the thing is that every god's followers and every spatial-region of Golarion seems to end up with a different story about what exactly happened in the Beginning of Things (as it is easier to say if you are forbidden from telling Merrin the real story, the one prominently featuring Asmodeus but not in a very Lawful Good way) and while they mostly tend to agree on points like Pharasma being the system's creator, anything past that quickly devolves into disagreement.
The most popular story has it that Pharasma was the last survivor of an older multiverse, and came here bearing something called the Seal by which she was able to bring about the present universe as a bubble of order within chaos, void, alienness, incomprehensibility, that surrounds it. The oldest and most powerful gods came into existence when, or shortly after, the universe was created; Asmodeus was one of those. Little is known of the oldest order; it shattered when something called Rovagug tried to destroy all of existence, some gods fought with Rovagug, other gods fought back, the gods fighting with Rovagug were destroyed and Rovagug itself was sealed - within Golarion, which makes this one of the most relatively important universes within the multiverse and a focus of attention for the greater gods.
The oldest and greatest gods, so far as Iker Egobar happens to know, have held their domains throughout recorded memory. For all he knows, they came into existence bearing those domains, those concerns, those goals. Younger gods, weaker gods, more often change their domains and concerns, though this is still very rare.
(They've decided not to mention Zon-Kuthon, the ancient god whose domain did change drastically after he traveled too far into the Void. It seems like the sort of thing that would cause Merrin to prioritize Nidal over everything else they'd prefer to have to prioritize instead.)