71 vs. 72 455

Was just wondering why the huge hp. rating drop from 71 to 72 455's was it an actual hp drop or just a change in how they are rated? (i have found myself a running buick 455 someone had stuck in a beat up 67 pontiac tempest... i ran the #s and it came out of 72 gs)

 

I've read it was a change in ratings.

from 'Brake HP' to 'Net HP' from 1971 to 1972.