Why did the couple recognize Rod’s voice on the radio immediately?

Early one morning, my husband turned on the radio. Suddenly he exclaimed, "That has to be Rod. No one else talks like that!" We listened attentively and heard the announcer say, "You’ve been listening to an interview with Dr. Rod Smith who has been telling you about a breakthrough in AIDS research that he and his colleagues have made."
    We both had Rod as a student and had agreed that he was absolutely the slowest student we had ever had. He spoke slowly, moved slowly, and wrote slowly. We suspected that he thought slowly. When he took an examination, he could never finish it. The unanswered questions kept him from having high marks.
      During his senior year, Rod announced that he wanted to go to graduate school, but no graduate school would accept him. He kept trying and a year or so later the department of biology of a large university allowed him to enroll in a few classes on a trial basis. They discovered that he was talented in basic research. He was then allowed to enter the master’s program, and later the PhD program. He cheerfully persevered. In six years he could proudly add the title of "doctor" to his name. His professional career began and we lost track of him, until we heard his slow, steady voice that morning.