Samsung chip division doesn't have guaranteed "buyer" or option to build SoC specifically for given product (even if usually you can bet on next generation Galaxy device as your target, you still have to design as if you were selling to open market - and that means less option to, for example, throw a lot of memory/cache resources that have been the main differentiator of Apple chips - which started from the same design as Exynos!)
Apple was actually the one designing those cores in the Exynos - up until and including the Exynos 4000. That was when Apple decided to sever ties with Samsung and stopped designing cores for them and Samsung had to go back to stock ARM designs with the Exynos 5000. That was why the 5000 series sucked so badly - Samsung lost access to those Apple/Intrinsity juiced cores. Itβs also what prompted Samsung to open SARC in Austin.
I'm wrong about that - it was the Exynos 3110 that had the Apple designed core that also appeared in the A4. And after that, it was back to ARM's humble designs for Samsung.
Hummingbird in SGS2 (this was before rename to Exynos) and A4 in iPhone 4 stem from cooperation between Samsung and Apple (who previously used Samsung SoC in iPhone 3G and I think in 2G). The teams split before both phones reached maturity in design. From then on Samsung kept iterating on the Hummingbird design till I think quite recently - I'm not sure if my Note 10+ doesn't still have cores derived from that design.
