What is the point of having a legal limit if you can get arrested still for being under it?

I was arrested on 9/9/15. I did have few drinks before I got behind the wheel. When I was getting off the freeway, I hit the curb where the off ramp was and got a flat tire. I pulled over and I had two friends in the car with me. My insurance is not triple A but my friend did have triple A so she called them. They showed up in a matter of minutes to help fix the tire. A few minutes later, a couple of officers showed up. They had asked me what happened and I told them. They asked me about drinks and I said yes and that I had 2. They gave me a few sobriety tests. He ended up arresting me saying that I was at a .086. We got back to the station and he said the I had to voluntarily do another Breathalyzer or a blood test. I went with the Breathalyzer and blew twice. I was at a .07. He said that I am not over the legal limit and that I would be released in 4-8 hours with no bail. Then he booked me and gave me to a police woman who was like oh no you’re going to be here until the morning with a $5,000 bail. My dad had called the jail as well and the told him they were keeping me for 72 hours. Eventually, my dad bailed me out, I never received a physical ticket/citation, a pink or yellow one. All I received was a print out of my booking info where I noticed it said DUI Alcohol/Drugs. I do not understand why it would say drugs. I did not have any type of drugs on me, or in me. They asked me if I took any drugs. I told them no. Why would it say that? What can I do if I was not over the legal limit?