Why is baseline testing annual?

Per current CDC Recommendation:
“If baseline testing is used, research suggests that most components of baseline testing be repeated annually to establish a valid test result for comparison. Baseline computerized or paper-pencil neuropsychological tests may be repeated every 2 years. However, more frequent neuropsychological testing may be needed if an athlete has sustained a concussion or if the athlete has a medical condition that could affect results of the test.”