Task completion rate increased from 53% to 73% but still under the standard average of 78%

A tree testing conducted for a website navigation and the result was 53% then, we redesigned the information architecture and produced a new tree testing with the new navigation. The result improved from 53% to 74% but it is still under the standard accepted rate of 78%. What reasons to put forward while communicating this? Is there anybody encountered this type of situation.