The big difference here is correctly summed up by Miss Nancy. The background has no angle of incidence on the light, while the ground has. The only time the ground will render the exact same colour as the background will be when you use the ortho camera from the top, and have a white light at right angles to the ground, so that there is no angle of incidence.

The best solution to this is to create a prop with an infinite curve, that acts as your ground plane and the background. This prop, when used with a universal light, will have colours that match closer, as well as getting rid of the horizon line.

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