First a little about me, I am not the type of person who accepts the answer ( that's the way it has always been done), it is not good enough for me in most instances, hence the challenge to the answer to the original question, it is obvious that there are, without knowing true figures but as a figure of speech about 50 % of the people that do the measurement from the fold with out experiencing any problems with customers.
As we all know anyone can write a book on what they think is correct theory, but then along comes a person who challenges that theory and rewrites a new book challenging the old saying of that is the way it is always been done. But I digress, I cannot see where a buckle can throw off a measurement by huge amounts, unless it is one of those huge cowboy turd kicker buckles, then it might change the equation. The constant that we know to be a constant in the belt world is the fold in the leather, the buckle will add a little extra in case the person has been hanging around the beer keg a little to long. That is my take on it but still left to confusion in the matter, and yes I have had it happen to me.
And the long time friend said to me that the measurements were right on yet the belt just didn't fit, but he nor I could put a finger on the issue, and it was hard to determine why, now this person is in Cali and I am in TN so it makes it kind of difficult to figure it all out, but he did also say that it was not off by much so I for now am going to attribute it to the belt buckle.