Outliers /PIS, as said before.
For one, both Wan and Korra (at these times) were using weakened Raava's power for their Avatar States. There's literally no reason for them to be "superior" to Raava when the strength of their Avatar States come from her.
Secondly, I'd hardly consider these stomp matches, like at all. Wan was getting rag dolled by Vaatu majority of that fight, even when Raava gave him power. They needed Harmonic Convergences power to be able to actually fight back, and it was only for a few seconds before sealing him. On top of that, Wan's attacks hardly did damage and were just staggering Vaatu so he could be sealed. The same goes more or less for Korra. Vaatu was able to restrain her for quite some time, he could harm her with his dark spirit beams, and Korras AS attacks just staggered Vaatu enough for him to be sealed (only to fail because of Unalaq).
And thirdly, most of all, this narratively makes no sense when these Avatars, at those times, were using the power of Raava in a weakened state who's far far inferior to Vaatu, especially one who became much stronger than he is normally. His influence over the dark spirits made him stronger and upset the light/darkness relationship to weaken Raava in return. There's no legitimate reason why they would be on his level at those points realistically and should be dismissed as PIS feats for the sake of the plot demanding them to hold their own against the main enemy.