Why do different bacteria ferment the same carbohydrate differently?

This depends on the nature of each bacteria metabolism.

Explanation:

Each bacteria has their unique metabolism, Not all bacteria have the ability to ferment carbohydrates, and not all bacteria can ferment the same carbohydrate, also one carbohydrate can be fermented into different products.

The most common carbohydrate is glucose. Glucose fermentation can lead to multiple fermentation products, depending on the bacteria, could lead to lactic acid, ethanol, formic acid, acetic acid, butyrate, etc. For example, Streptococcus lactis and Lactobacillus spp ferment glucose into lactic acid. Escherichia coli, Salmonella, Shigella and Proteus species ferment glucose into different products like formate, acetate, lactate and succinate.

This is because each bacteria has its own growing requirements, and is useful to distinguish each bacterium from another based on the carbohydrate fermentation pattern.