Because you are only interested in the bottles that Julio samples. Of the 20,000 bottles produced every day, Julio only samples 2,880 of them. He assumes that if over 1/50 of his sample is faulty then over 1/50 of all the bottles will be faulty and so he rejects the line. In his sample, 1/50 of the bottles is 57.6, so the lowest whole number that is too high is 58 and so he must reject the line if he gets 58 defective bottles or more.