Why is autism now called a spectrum disorder?

The term “spectrum” refers to the wide range of symptoms, skills, and levels of disability in functioning that can occur in people with autism spectrum disorder (ASD). Some children and adults with ASD are fully able to perform all activities of daily living while others require substantial support to perform basic activities. The Diagnostic and Statistical Manual of Mental Disorders (DSM-5, published in 2013) includes Asperger syndrome, childhood disintegrative disorder, and pervasive developmental disorders not otherwise specified (PDD-NOS) as part of ASD rather than as separate disorders.

Source: http://www.ninds.nih.gov/disorders/autism/detail_autism.htm