Deductions & credits

I entered the $2400 and the $1200. AGI was $21k under the joint limit. TT incorrectly gave us a $1200 credit, which I didn’t question. All I remember doing at the time I did my return was verifying the payments. 
I paid the extra amount to IRS but am puzzled why TT gave us the credit in the first place since we did get our two rounds of full payments.