Min rate has 2 projections and 4 purifications every 10 cycles, which requires at least two glyphs of purification. Having each purifier perform 2 purifications means you have 3 glyphs each producing 2 copper, which forces one of the pipelines to be split for 4 pipelines making 2+2+1+1 products. Having one of the purifiers handle 3 purifications and the other handle 1 allows for a 3+2+1 split, which only requires 3 pipelines. The 3 product pipeline has some extra cost since it needs to run at period 3, but this is mitigated by getting good arm-reuse out of the forced track-arms and hexarms.