How is it possible that from the millions of Clones made to serve thr Grand Army of the Republic only 1 was classed as defective? Surely all the ones made at the same time as him should have also been defective.

With numbers ranging as large as 1,728,000,000 clones made your really going to tell me that there was only 1 who wasn't perfect. That's a failure rate of 5.78703704E-8% which is just insane.

Reference for that number. https://starwars.fandom.com/wiki/Grand_Army_of_the_Republic/Legends