Question on RMD and 10% withheld

Hi,

My mom passed in 2019 and I inherited her Annuity. Each year since, I have taken the RMD with 10% withheld for federal taxes.  When I input the information into Turbo Tax this year, my refund was reduced an additional $355.68!  

Gross Dist:  2543.23

Fed Inc Tax withheld:  254.32

 

My income did not put me in a different tax bracket, so I am confused as to why this happened. Any help appreciated! Thanks!