Others can correct me if I'm wrong since I'm on TRT and was never really a PCT expert, but stimulating too much test production can also stimulate too much E2 production, which will shut you down once you stop taking the SERM if E2 is still elevated. That would result in a failed PCT (at least temporary failure).
Tapering down to 20 or 10 eliminates this problem, but the higher amounts are just unnecessary IMHO, though some people still like the 40,40,20,20 method and have good results with it. Though some evidence would suggest that a longer, lower dose PCT might be more effective. 20mg is well within the therapeutic range of this drug; in other words, at 20mg it's already doing it's job.