b_spinoza
However, "at least every 24 hours" is also perfectly acceptable and idiomatic English. It is very common in English to use this construction, "at least every X time period," to mean "X often or more often." If you say, "I make sure to call my distant relatives at least every year," you mean with that frequency or more. If you say, "I make sure to stand up and stretch at least every hour," you mean with that frequency or more. If you say, "these machines are required to reboot at least every 24 hours," you mean with that frequency or more.
In other words, "at least" does not qualify the number of hours (such that more hours would also be acceptable), but the frequency (such that greater frequency would also be acceptable).
Hopefully this response helps you to understand why English, fickle language that it is, works this way in this case.
However, it is also important to recognize that they were brought together and redacted by an editor (or committee of editors, perhaps even spread out over time) that arranged and tweaked the stories. At the very least, it is easy to glean from the text that they are intended to form a whole "collection," with connections between the various myths. The textual evidence suggests that the connections are stronger than that.
So while it is the more common error to underappreciate the differences between the accounts, it is a less common but equally erroneous mistake to underappreciate the "unity" (of a sort) that they have.