Perhaps in this fanfiction's plot, Sakura has some sort of special inheritance or power-up. It could be that she has a connection to an ancient and powerful force. This power could be something that was dormant within her all along and finally surfaces, making her stronger than Sasuke. For instance, she might have a link to a long - forgotten sage or a special kekkei genkai that was previously unknown, which now gives her extraordinary abilities.
It could also be that the writer wants to subvert the traditional power dynamics. In the original series, Sasuke is often seen as very powerful. But in this fanfiction, Sakura might be stronger because she has a stronger willpower. She could be more determined to protect her loved ones and this gives her an extra boost in strength. For example, her love for the village and her friends drives her to train harder and unlock new levels of power.
One possible reason could be that Naruto has a stronger willpower in the fanfic. His never - give - up attitude might be enhanced, allowing him to push past his limits more easily than Sasuke. For example, he might have an even deeper connection to the Nine - Tails' power, enabling him to draw on it more effectively in battles.
One possible reason could be that Naruto has developed some new and extremely powerful jutsu in the fanfiction. Maybe he has found a way to combine his Sage Mode with his Kurama chakra in an even more efficient and powerful way that gives him an edge over Madara.
One possible reason could be that Naruto has access to some new and powerful jutsu. Maybe he discovers a secret family technique or is taught by a very powerful sage. This new knowledge gives him an edge over Kakashi.
The relationship might become more equal. Since Sakura is stronger, Sasuke can't look down on her as he might have in the original story. They could become more like true partners, respecting each other's abilities.
Protectiveness. Sasuke has seen a lot of danger in his life. He knows Sakura is a valuable person, both as a friend and as someone he cares about. So his possessiveness could stem from his desire to keep her out of harm's way. He doesn't want any bad guys to get near her.
One reason is that Naruto has the Nine - Tails' chakra. In fanfic, this power can be further enhanced or manipulated in ways that make him far more powerful than Sasuke. Another reason could be his more outgoing personality. He might be able to form alliances more easily, leading to him having more resources and power - ups at his disposal.
One possible reason could be that Sasuke is too focused on his revenge. His single - minded pursuit of power and vengeance against Itachi leaves little room for romantic feelings. He might see a relationship with Sakura as a distraction from his goals.
One reason could be that in some fanfiction, the characterizations of Sasuke and Sakura might be off. Maybe Sasuke is portrayed as overly cruel to Sakura in a way that doesn't seem true to his character in the original series. Another reason could be that the storylines in these fanfictions are too clichéd, like always having Sasuke constantly reject Sakura in the same uninteresting ways.
In fanfiction, Sasuke might snap at Sakura due to miscommunication. Sakura could be expressing her concern for him in a way that he misinterprets. For instance, if she tells him not to go on a dangerous mission and he thinks she is trying to control him instead of just caring about his safety. Another factor could be the influence of external forces. Maybe there are other characters or events that put him in a bad mood, and Sakura just happens to be there at the wrong time, so he snaps at her.
One possible reason could be Sasuke's continued obsession with his revenge - even after they got married, his focus on past grudges might have driven a wedge between them. Another could be differences in their life goals. Sakura might have wanted a more settled family life, while Sasuke was still drawn to his wandering ways in the name of protecting the village from afar.