Discount in premium is tricky as Insurance Industry is regulated by IRDA, there are some products like Third Party Motor Insurance for which the premium is decided by IRDA for all Insurers. In these products Insurance Companies does not have the right to give discounts (even if they wish).
But other products like Own Damage Cover in Motor Insurance, Insurance companies provides Discounts.
You will have to compare same product with different Insurers as they have different campaigns at different times.
