The difference is not a significant one. The way approvals and licensing are handled doesn't allow much variance. The testing is primarily handled by the additive companies who put together add packs and base oil blends to send off for testing to that license or approval. If it passes, it's sold as an approved add pack. Brands / manufacturers purchase those add packs, buy the license, and then blend the oil to the recipe of that test. Blenders are limited in how much they can deviate from that recipe. Something like adding extra anti-oxidant, a little ester, or some ashless FM are out of the question. Even though they would improve the oil, they weren't present for testing and thus not allowed.
There are multiple approved add packs out there with some being better than others, but not by any huge margins. The most variation you'll find is with Shell and Mobil 1 as they co-own their own additive company.
This corners licensed oils into the same small box chemistry wise. You often have to ditch the cert to go outside the box to make substantial leaps in performance. The certs offer no incentive to do more than the minimum standard either. Therefore, it's usually a race to the bottom with brands spending more time finding ways to cut down production cost so they can retail cheaper than a competitor.
