Why has my first xbox offense resulted in an 8 day suspension?

Suspension lengths can vary based on the severity of the situation. So not all first time suspensions are 24 hours. They can be anywhere from 24 hours to permanent.

your xbox has a device privilege suspension lasting 60 days on first console offense

There are no three day suspensions. 24 hours (first offense) then 7 days.

Multiple suspensions will result in a permanent ban, but there is no exact number. Depending on severity, you can receive a permanent ban on the first offense.

I got a communications ban 3 weeks ago for 4 days. Logged in this morning and they gave me a 22 day ban for the exact same thing and I wasn't even on my Xbox yesterday. I pay money for this account and shouldn't be banned for no reason. There is no one to speak to. Why are they banning me and additional 22 days for something I already did my time for?

The majority of bans/suspensions arent given on the same day as the actual offense so whether you were actually online at the time they gave you the ban is irrelevant.

What likely happened is when you were serving your first suspension they found the offense that caused your second suspension and added that one on so it would have been for a totally separate offense.

The only ones who handle bans/suspensions are the enforcement team and the only way to reach them is via a case review at https://enforcement.xbox.com