Layer time, most likely. You can avoid it by setting minimum layer time higher. When the curvy part is being printed, it takes longer for that layer, so it gets to cool down more before the next layer, in contrast to still being a bit hot before the next layer if the layer time is short. This causes unevenness in the looks a bit. You may also decrease the effect by lowering the printing temperature, but I would not recommend it. Also, part cooling fan can have an effect as well.