Why is it sometimes difficult to diagnose appendicitis?

CAT SCANS help. In the modern era, ct's of the abdomen have been very helpful for diagnosing appendicitis for the non-classic presentations. In the past, it was not uncommon to operate for appendicitis only to find a normal appendix 10-15% of the time. Thanks to ct scans, this 'negative appendectomy' rate has decreased. Of course, there are radiation risks associated with ct scans' so we have to be selective.
Everyone's different. In my experience, about 1/2 of all people that i see with appendicitis come in with classic signs and symptoms that make the decision to operate quite easy. However, there is a lot of variability in the description and location of the pain, as well as other diseases that can resemble appendicitis. Added to this is the fact that we want to operate as soon as possible, before the appendix ruptures.