Why are two 2204 2300kv motors from different brands not the same?

We all know that kv defines how fast a motor spins in relation to the voltage, or RPM = kv * v. If so, why are two 2204 2300kv motors from different brands not the same? Why is a cobra 2204 2300kv diferent from an Emax 2204 2300kv and Sunnysky 2204 2300kv?

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