Enchanting is done with a random stat bonus. It looks like the bonus might be from 30-50. But if you use the same type of raw focus, and enchant multiples of the same type of item (1 chestplate for each raw focus) using the same stat and same type of raw focus, you are still going to see these random numbers changing. One sandstone might do 37, another 45, another still 42, another 48, etc.