You get the multi-bike discount through the difference in the two values in the quote calculator - at home, and away from home. The bikes you take away from home are rated more highly (cost more) than the bikes you leave at home.
For example, I own 4 bikes, but I only ride one bike away from home. My total value at home is £7,500 but the maximum value away from home is £2,000.
If I insured all my bikes away from home (i.e £7,500) then I wouldn't get a discount. However, as the maximum value away from is only £2,000 then this results in a multi-bike saving of around 50%.