Why does ishmale assert that only one in ten thrown harpoons finds its target?

Check out this Quote from chapter 62,

"But however prolonged and exhausting the chase, the harpooneer is expected to pull his oar meanwhile to the uttermost; indeed, he is expected to set an example of superhuman activity to the rest, not only by incredible rowing, but by repeated loud and intrepid exclamations; and what it is to keep shouting at the top of one's compass, while all the other muscles are strained and half started—what that is none know but those who have tried it....No wonder, taking the whole fleet of whalemen in a body, that out of fifty fair chances for a dart, not five are successful..."