Even when manufacturers have the provision of offering supplier discounts, a few suppliers are eligible for certain discounts while a few are eligible for the others. Also, a certain supplier might get a discount for a particular model of the good while some other supplier might not get it for that particular variant or model of the good or product. Also, depending on the relationship of the manufacturer or company with a particular supplier, i.e. depending on how long their relationship has been going for, depending on the trust levels they have between them, and depending on how much business a particular supplier brings to the company, each supplier may have varying discounts.
For example, General Motors offer discounts to their suppliers for their cars and other vehicles from Chevrolet, Buick, Cadillac, GMC, and other GM companies.