170 vs. 225

What was the cause of the big dropoff in HP on the 225 from 1971 to 1972? Or was that just a different power rating system causing the number to drop? I read through the allpar article but didn't notice an explanation for this. In other words, in looking for a transplant or engine to build, is there any actual reason that a '71 or earlier 225 would be better than a '72-up one?