Eggman had little time to mass-produce her model in Sonic 1, this Splats being the last one of a batch made, production halting right in the middle of her paint-job. She properly activated via Orbot just running tests on scrapped badniks, then feeling too guilty to shut her down.

Splats since spent years upgrading and training herself in a bunker factory but kept her colors thinking she was always a special model like Metal that just didn't get a chance. Despite once being a common badnik, she might be capable of what she once only believed she could do.

Simply put, Splats swears Metal was used instead of her to be Sonic's robotic rival because "she'd make getting Sonic too easy" In reality, Eggman saw it more fit to use a hydraulic press as a final boss in Sonic 1 and didn't even think of her before developing Metal later.