Inliners International Forums General Discussion Engines 250/155hp vs 250/145hp vs 250/100hp - differences?

I am looking at the ratings of the stock 250cid over the years, trying to figure what changed.
250/155hp, 1966-1970

250/145hp 1971 -
what changed? Smaller carb? Other?

250/110hp 1972
I know this is a NET rating instead of GROSS, but any other changes vs 1971?

250/100hp 1973-74
Loss of 10hp at least in the rating.
Changes? Retarded cam? Other?

I don't have enough manuals from this era to figure it out, thought somebody might know what changed in these years.