This idea has probably been raised before, but I have an idea of how Garnet's future vision might differ from Sapphire's. Put simply, Sapphire can only see the events of one possible timeline, while Garnet can see more than one, if not all of them (though obviously not all at once in the latter case, as demonstrated in "Cry for Help"). In "Keystone Motel", and prior to the events of "The Answer", Sapphire is shown to see one strict sequence of events, and as a result is firmly convinced that it's an accurate glimpse into the future. Garnet, however, has stated that she can see time splitting off into different paths and generating different outcomes, causing her to still rely on future vision at times, but acknowledge that it needs to be taken with a grain of salt. (Although this raises the question- given both how Ruby first altered her fate and her subsequent time as Garnet, why was Sapphire so firmly convinced about the certainty of the future in "Keystone Motel"? It seems like an extremely odd case of regressive forgetfulness.)
Thoughts?