PM -Learning Rate Question - Revision Kit doubt

A company predicted that the learning rate for production of a new product would be 80%. The actual learning rate was 75%.The following possible reasons were started for this: A)The number of employees new recruited was lower than expected. B) Unexpected problems were encountered with production. C) unexpected changes of safety and health laws meant that the company had to increase the number of breaks during production for employees. Question -Which of the following could be a reason for the difference between expected learning rate and actual learning rate? The answer to this is option A only. Can you please explain how so as this was asked in June 2015 exam.