A sample size as small as 250, or even 400 shipments, isn't sufficient to establish correlation between string tension and something as rare as a neck breaking during shipping.
You're attribiting the fact that none of your shipments have experienced neck breakage to the fact you always ship instruments detuned. But there's nothing in what you've done to establish they're related so far. Your sample is too small.
Your opinion is using logic similiar to the thinking of the guy who always kept a mongoose in his city penthouse - to keep the cobras away. When friends pointed out there were no wild cobras in the city, the man with the mongoose smiled and exclaimed "Really? Wow! This little mongoose is even more effective than I thought it would be!"