Does anyone know why a dwi misdeamnor would be reduced to a dui city ordiance?!

I got charged with a dwi misdeamnor, then the next day i was advised it was reduced to a dui city ordiance. My arrest was strange. I was arrested by an off duty dectective who said i drove the wrong way and he said he thought i was dwi so i go to police station and they asked for a breath test. I said no, then i was taken to the jail.
I feel this could be thrown out. Any ideas? Thanks!