The One X has a unique hardware set, it's a third model effectively as it has its own CPU/GPU/RAM not shared with either the other One models or the Series X. It's easier to maintain the S line, then create a new "middle tier" model that's a clocked down or removed parts Series X but is the same hardware set inside.
Xbone S: Jaguar CPU, GCN2 GPU, DDR3 RAM
Xbone X: Jaguar Evolved CPU, GCN4 GPU, GDDR5 RAM
Series X: Zen2 CPU, RDNA2 GPU, GDDR6 RAM
Presumably the Lockhart would also be Zen2/RDNA2/GDDR6 but just with "less" of everything.