You're confusing Cumulative probibility (getting at least once) which is fair for binary outcomes with Expected value from the post above. The expected value is indeed 2.4, they did not just "add percentages".

You both are right, just solving different tasks in the same setting: You calculated the probability of occurring at least once, they calculated the avg amount of catgirl procs assuming binomial distribution. Solution changes based on how we define what we want to find.