Something is not making sense with out of shield. Need some help.

My main is Lucas.

When playing against Cloud, why am I unable to punish his back air on shield but he can punish my forward air on shield with up b? Looking at ultimate frame data, Lucas forward air in -3 on shield, at worst, and his up b is frame 7. Every time I touched his shield he could up b before I could even touch the ground.

But when the reverse happens and he touches my shield with back air, which is also -3 on shield, he can immediately throw out another option before I can neutral air, which is also frame 7.

Why is Cloud able to punish me but I can't punish Cloud when the frames are the exact same?