It's all a mystery how they determine the rates it seems.
I was with Aviva for about 10 years with my home and car and about 5 years ago the broker I used couldn't offer bike insurance through Aviva just the home and car. I switched to Dalton Timmis and kept the home and car in place and added the bikes.
Each year the bikes and home policies crept up with my car coming down as I retired and didn't need the business insurance on the car any longer just regular insurance, so it came down a few hundred.
Riders Plus I spoke to directly and they were cheaper on the bikes alone with no car or home policies required when I switched a few years back after shopping around. I don't think they write those policies anyway. Last year I had CAA quote on the car and home as the home again kept moving up with Aviva and CAA was $350+ cheaper, so I switched.
I hear what your saying, yet I was a long time Aviva customer with 0 claims and a clean abstract, so who knows how they arrive at their policy premiums.