Trick of mind

Hi, could you please tell me what is the meaning of "trick of the mind" in the following text?
When Ann brought up the bad quality of water and its relation to leukemia again, he told her about the experience he'd after buying his car, a Volvo, that they were riding in. He had suddenly noticed Volvos everywhere he looked. But that had been an illusion he said, a trick of the mind. There really weren't a lot of new Volvos around; he'd simply become more aware of them. He suggested to Ann that she was experiencing the same sort of phenomenon with leukemia.

Thanks in advance.