The biggest variable, I believe, is the skill and experience level of the doctor interpreting the echo. Right after that is the skill and experience level of the technician who does the study. These things (echo's) are quite variable, and if a particular anomaly is very subtle, it may be missed by either person. The more pronounced a defect is, the less likely they are to miss it, but they may mis-judge the severity.
As an example, I have a very small ASD (atrial-septal defect). The defect causes me no symptoms and hasn't changed over all the years I've been having echo's. I have probably had it since birth. Some echo's mention it, others do not. It doesn't go away, but some echo's might make the novice think so.