My federal tax due is $406 but I filed 0 exemptions all 2021. why do I owe money at all?

I'm looking at my "FITWH" and the filing status just says "S" and below it I see my state filing status that says "S" Exemptions "0" my federal taxes withheld just has an S next to filing status. there is no zero. 

( I ONLY HAD ONE JOB LAST YEAR )