The first and most obvious reason is your battery could be old. Five or more years for a car battery is a good life. Car batteries are primarily only used for cranking the car engine so they should last long since the depth of discharge is typically low which reduces the amount of cycles they experience. However as they age they experience a more rapid falloff of capacity and eventually go flat faster which is why older car batteries will start to die more often as they age.
No two car batteries perform alike. Why? Well on paper they do, however the real reason no two batteries perform the same is because they are subjected to different environments. Cars have different starting requirements and accessories that may need powering when the engine is off. Ambient temperatures have significant effects on car batteries and temperatures vary around the globe. The biggest reason batteries perform different is due to the users. Assuming the car battery is used for a typical car with a combustion engine, the user will have their own unique patterns such as frequency of starting the car, driving distance, non-driving durations and accessory use when the car is not running. This is why there is a large fuzzy area in the real life performance of car batteries.