There is a theory that every time it is possible that two different events could have happened, the universe splits into two universes, one where the event occurred, and one where it did not. As such, there would be as many universes as there were possible states of that universe.

In our universe, the fall of Dalamud was not prevented, precipitating the calamity and all the events of 'a realm reborn' that we've participated in. In another version of Eorzea's timeline, perhaps Dalamud never fell, and everyone there is still in a blissful Astral era.

If you wanted to, you could perhaps consider the power of an Astrologian not so much as 'reading the future' but choosing, out of a possible set of futures, which future to bring about.

The stars have long been thought to affect our futures, or sometimes even determine them, so it is natural for an Astrologian to turn to the stars to borrow that power. To ask the stars to show them what their choices are, perhaps. As an Astrologian becomes familiar with the stars, they are granted knowledge of a wider range of possible futures. By learning which stars affect what aspects of the future, they have greater control over which kinds of future to peer into. Astrologians never reach the ability to tell /any/ future they desire, and an element of randomness always remains even as their ability to manipulate that randomness grows. The future always remains a place where anything is possible.

While most classes have to play the hand they're dealt, or move forward with the possibilities presented to them, the Astrologian can change exchange what is possible for a different more favorable set of possibilities, if the stars and cards will show them that set for them to choose.

Nobody knows how magic really works, and that's what makes it magic, but, however they're doing it, Astrologians write the futures they want. They read ahead in the story a little bit, and edit it to suit their wishes before it actually happens.

In one Eorzean universe, your tank was killed by the boss when a tank buster went off and they couldn't withstand the damage. The Astrologian saw that coming and edited the event so that the tank survived instead. Most of us have had a few days where everything just seemed to go right. We call it luck or good fortune. An Astrologian nods to him or herself, thanks the stars, and calls it a job well done.