I am not entirely sure I understand what you are asking but will give a go as if I did
Let's say your initial certification would require 6 mandays of auditing. After you have the certification audit performed (6 mandays) you will then have periodic recertification audits. The recertification audits would take 4 mandays (2/3*6 mandays), assuming your employee count didn't change from the initial audit. However, we are using the initial certification of mandays to determine this as if nothing has changed (and it may not have.)
The 2nd part is saying that when it comes time to recertify, you need to recalculate what it would take (regarding mandays) to audit based on the current state of the company, not what it was when you initially certified. So you would look at your number of employees, locations and determine how many mandays it would take for an intial certification, THEN take 2/3's of that and that would be the mandays needed for the recertification audit.
While it might not make sense to those whose situations (employees remain the same, locations remains the same) think of the situation where a company goes through a downsizing of employees. The current number of employees might result in a lower number of mandays needed to audit. (Same could be said of a location increasing their number of employees, thus resulting in additional manday(s) needed to audit.)
If I have misunderstood, disregard my post