In his excellent post, Who Controls Bitcoin Core?, James Lopp describes Bitcoin Core as “a focal point for development of the Bitcoin protocol rather than a point of command and control.”
These numbers may be interpreted as centralization of power in Bitcoin. However, peering into the processes governing the code, one realizes that they are quite decentralized. A system of checks and balances prevents a single person or group from making arbitrary changes to the protocol.
Bitcoin Core may be the focal point of Bitcoin development, but its code is decentralized.
Turning now to Ethereum, one sees a different situation.
Ethereum has many clients, two most popular being Geth and Parity. Here, the processes which govern the code of these repositories has less decentralization. The Ethereum Foundation and Parity Technologies hold much of the control over code commits to their respective clients.
In contrast to Bitcoin, the focal point of Ehereum development is the protocol itself and decentralization is shifted to the different client implementations.