Insurance premium increased for removing one of three vehicles?

I use Progressive. I had three vehicles on my policy. One new car and a couple older ones. I removed one of the older ones today because I sold it. When I did my insurance premium INCREASED by $57. I didn't change anything else. It didn't make any sense. I called them and they just told me it's because I lost my multivehicle discount. Even then it makes no sense that it increased with less vehicles on the policy. Can someone wrap my head around this???