I haven't seen the code, but maybe c&p-ing the C entrant into the C++ box to get performance parity would make the C++ entrant run afoul of typical C++ coding conventions.

(You can then have a debate about whether you want to see idiomatic C++ in benchmarks or purely the most performant C++ possible. Personally if I'm shopping for a language for a project then I'm far more likely to be interested in the former.)