It's a hell of a lot easier to rate great individual performances than it is to rate great horses.
Circumstances dictates outcomes in horse racing - inferior horses favored by the wide variety of circumstances beat superior horses every day - and it happens at every level.
Also - horses develop and go in and out of form at different times - they obviously have preferences to a wide variety of things starting with surface and distance.
Rating a performance is much easier and can be done with a much higher degree of accuracy than it is to rate how good a horse is overall.
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