Some “theories that can explain everything” may actually have the property that they can explain any individual observation but constrain what combinations of observations we can observe.
Consider, for instance, a vague but strongly adaptationist version of evolutionary psychology: it says that all features of human thought and behaviour have their origins in evolutionary advantage. Pretty much any specific feature of thought or behaviour can surely be given some sort of just-so-story explanation that will fit this theory, but it might be that feature 1 and feature 2 require mutually incompatible just-so stories, in which case the theory will forbid them both to occur; or at least that no one is able to come up with a plausibly-compatible pair of stories, in which case the theory will predict that features 1 and 2 are unlikely to occur together.
Some “theories that can explain everything” may actually have the property that they can explain any individual observation but constrain what combinations of observations we can observe.
Consider, for instance, a vague but strongly adaptationist version of evolutionary psychology: it says that all features of human thought and behaviour have their origins in evolutionary advantage. Pretty much any specific feature of thought or behaviour can surely be given some sort of just-so-story explanation that will fit this theory, but it might be that feature 1 and feature 2 require mutually incompatible just-so stories, in which case the theory will forbid them both to occur; or at least that no one is able to come up with a plausibly-compatible pair of stories, in which case the theory will predict that features 1 and 2 are unlikely to occur together.
Arguably all theories are actually somewhat like this.