Sum insured is reduced, premiums increase

I recently received my insurance schedule for the annual increase. I informed Aon that I wished to reduce the sum insured on 2 properties and received an amended schedule. The sums insured were reduced but the premiums INCREASED. I remembered that I had a problem the previous year which took about a month to resolve - it was the same problem. Is there no connection between \sum insured\" and \"premium\"? I am starting to think that the premiums are arbitrary and have no relation to risk or value of item insured. This is very scary."