It is entierly up to RNG
Currently the banners right now are gurateed to always give you the featured mythic everytime you hatch a mythic egg
But usualy its very much up to luck, you have a higher chance of hatching the mythic thats featured but the mythic egg has a small chance of being an off banner mythic
So its all up to weather or not you are lucky, banners do reoccure every 3 months so you always have more chances