A mockup is a sales asset, not a proof
A mockup shows a design in ideal light on a garment that drapes perfectly. A proof shows what a specific press will put on a specific blank. Direct-to-garment printing shifts colour and behaves differently on every fabric blend — a 100% cotton tee takes ink differently from a 50/50, and neither looks like an RGB screen. Use mockups to judge whether a design reads and suits the garment; use a sample to judge colour.