I'm not asking about them as nouns, or the calculus and mechanic meanings of 'differential'. ODO doesn't distinguish them, and synonymizes 'differential' with 'difference'.
In 1 beneath, why not use 'different'? What'd change?
Why not use 'differential'?
Gender has traditionally played a key role in the pricing of insurance policies.
In 2004 an EU Directive prohibited all discrimination based on sex in the access to a supply of goods or services. In principle the Directive therefore prohibited the use of gender as a method of determining insurance premiums and benefits with regard to contracts that were entered into after 21 December 2007. However, the Directive also stated [1.] differential pricing could be maintained where statistical evidence supported such an approach. Insurance companies regarded this as crucial because as women drivers are statistically proven to have fewer accidents than male drivers, premiums for female insurance policies have generally been lower. In a similar way, because women live longer, men have traditionally received a higher rate from their pension annuities because their life expectancy is lower and as such their pension savings are able to produce more income over a shorter time.
Such continued practice of price differentials was subject to a review after the Belgian Constitutional Court asked the European Court of Justice to assess the validity of differential pricing. This in turn resulted in the ECJ ruling on 1 March 2011 that insurers cannot charge [2.] different premiums to men and women based on their gender from 21 December 2012 onwards. In the case of car insurance, the significance of this ruling is that female car insurance premiums will rise while male insurance premiums will fall. And where high costs of insurance have in the past discouraged young men from buying fast cars that are more likely to lead to road accidents, a reduction in insurance costs could worryingly result in greater purchases of high performance cars.