In fairness, misdiagnosis of appendicitis is largely down to utilisation of technology. Most modern institutions utilise imaging (ultrasound/CT) and this results in a high specificity for appendicitis (especially CT, which doesn't suffer from the user variability that ultrasound does).

Old school surgeons will insist that they can reliably make the diagnosis without imaging (and that's probably where you get a 20% false positive from). Some institutions will push this notion too - either to reduce imaging expenditure or increase surgical turnover (sad but true).