Hey there,
When you choose a Legendary outfit through Sure Egg, if the Legendary outfit you obtain from Surprise Eggs is not the one you chose, you will gain 1 point for your Sure Meter. When the Sure Meter reaches the upper limit, the next Legendary outfit you get in Surprise Eggs is guaranteed to be the one you chose through Sure Egg. Once you have obtained your chosen outfit, regardless of whether the current Sure Meter has reached the upper limit, it will be reset to 0.
If you do not choose any outfits through Sure Egg, then you will not accumulate Sure Meter points.
You can change or clear your Sure Egg choice at any time. However, once you change or clear your Sure Egg choice, your Sure Meter will be reset to 0.