My understanding is that in the recent past, both Chinese and Japanese rules used 5.5 for komi. People generally came to the consensus that 5.5 was a bit too small leaving white at a disadvantage, which was observed in pro game statistics. Hence, a logical response was to increase komi by a point.
However, with area counting (Chinese rules), most games end with an odd score difference (on the board before komi is applied), since the exception requires a seki. Hence, a 6.5 komi would not be much different than 5.5 komi under Chinese rules (B+5 still loses, B+7 still wins, and B+6 is unlikely). Thus, for Chinese rules, the komi was increased to 7.5 to make a more substantial difference.
To rephrase your last sentence: playing a Chinese game with 7.5 komi is (almost) like playing a Chinese game with 6.5 komi.