because not all PSU are equal with many failing to achieve their stated power. the recommendation is to cover the lowest quality of the PSU field. The fact is any quality PSU with 22a on its 12v rail will run the your system with ease. the antec EW 380w is one example that is below 400w in stated power (although easily breaks 400 in peak power) but can power that system with no problem.
*also note the thread in question is SAAIELLO's and represent his views on power recommendation. I'm sure somewhere in his thread he explains how he got to those figures.