An 11-year-old girl with ADHD combined presentation has completed an adequate trial of methylphenidate extended-release, titrated from 18 mg to 54 mg daily over 8 weeks. At each dose, her teacher and parents report only marginal improvement in sustained attention and impulse control on standardized rating scales. She tolerated the medication well without significant side effects. The PMHNP recommends switching to mixed amphetamine salts extended-release rather than adding a second agent. Which of the following best explains why switching stimulant classes is a rational next step after methylphenidate failure?