That's because a condition doesn't have to be previously diagnosed to be considered pre-existing. 

Under Australian Private Health Insurance Act (2007), a condition may be considered pre-existing if signs or symptoms existed in the six months before you joined or upgraded your hospital cover — even if you didn’t know what the condition was at the time.  

Also, these signs and symptoms should have been reasonably apparent to either the member, or a reasonable general practitioner had the member been examined in this 6 month period;  

Because of this, insurers are required to assess medical information to determine whether those signs or symptoms were present before your cover started and whether the pre-existing waiting period applies.