Quick question about name change..

see the funny thing is, my mom is the one that told me my maiden last name becomes my middle name because that's what she did..

but when i went to change my name on my drivers license and social security they had left my middle name.. and i was like oh no im going to use my old last name..

and all my bank and credit cards have me as keeping my middle name.. so im guessing here where i live they keep the middle name and jsut drop the last name.