Hmm, interesting phenomina.

Receivers don't assign different centers, but they do have to work with a signal of very low bandwidth due to the narrowbanding of transmitters some years back. As a result, the slope of the signal sent by the transmitter when turning on and off was decreased, and the internal circuitry of the receiver that determines where the on and off edges of the signal exists in time are very depended on the filters inside the receiver. Thus servos could center different with different receivers. Even with receivers of the same make and model could experience this.

Thanks for pointing that observation out CP.