Bacteria can be quite diverse even amongst the same species. Bacteria in the same species can have difference virulence factors, which describe genes that cause disease and evade host defenses. For example, one isolate of group A strains can have genes that code for Streptococcal toxic shock syndrome which causes severe shock. Most strains do not

There’s some evidence that nec fasc is caused by group a strep with certain proteases. It probably also has to do with what body part gets exposed. The mouth and GI tract is super good at fighting bad bacteria

Basically it’s largely due to different strains of the same bacteria. Same reason there is antibiotic resistance