1. If you really went through five new tubes during a change before you found one that held air, I would consider the possibility that there is an issue with your tire mounting technique, and that you are putting holes in your tubes with your levers. I have gone through a lot of tubes over the years and have never had a single new one fail at a seam, only at the valve. Perhaps you bought them all at the same time and got a bad patch, but you say there was another occasion where you went through three new tubes. I have worked in a shop and find it quite hard to believe that you have had such bad luck.
2. Regarding your exchange with Retro Grouch, obviously the notion of statistics means something different to you as compared to most people. That's ok, people think in different ways, but I suggest you stick to adjectives in the future.
+1
also
the idea that most flats
are caused by a seam bursting
is complete nonsense