Explain to him/her that random selections are generated from an unbiased computerized selection program. As such, every employee in the testing pool has an equal chance of being selected during each selection period. Some employees are selected multiple times during the calendar year, while others may not be selected at all.
An employee can be randomly selected multiple times during a calendar year. Employees have an equal chance of being selected in each selection period, regardless of prior selections. See this post for details.