Ci porti vs Ci porta?

I am a little confused about the imperative informal use of “bring us” - Ci porti… (informal) and ci porta (formal).
The word Portare is ending with -are and according to grammar - informal way to use imperative form of verbs ending with -are is -a, and formal is -i.
So why is the “Ci porti” (informal) and “ci porta” (formal) are opposite in the course?

(Italian 1 - topic 12)

I’ve answered that in the other thread, see here: