In short: Benchmark numbers were always more important than consistency (traditionally, GPU vendors competed on frames-per-second, not on image quality). E.g.

In short: Benchmark numbers were always more important than consistency (traditionally, GPU vendors competed on frames-per-second, not on image quality). E.g. floating point behaviour is only the tip of the iceberg. Every GPU vendor also does mipmapping differently (to balance image quality versus required memory bandwidth).