The intention behind that rule is so that the size of the handicap itself is the same, whether playing with territory scoring and with area scoring, not so much that the absolute difference itself is important.
Since chinese rules give those extra point but territory does not, an N stone handicap is “N points larger” when using chinese score. According to a (completely unreliable but let’s take it to be ok for the sake of the argument) classic rough estimate, “a handicap stone is about 10 points worth”. So, “8 handicap chinese” would be almost as large as “9 handicap japanese”. I assume that specially if you are using handicap games for ranking computation, this difference complicates the math and would be confusing to people.
The “traditional” solution is to subtract points for chinese handicap, so that it gives the same result as the japanese. Alternatively, it would have had a similar effect it one point per stone had been added to the japanese handicap, so that it gives the same advantage as the chinese one. It is simply the case that “Japanese tradition” won, and so Chinese handicap score is adjusted so that it is equivalent to the Japanese