Has to do with initial servicing. If the battery was serviced and charged properly you can just put it in. If it was a brand new battery, just filled with acid, it needs to sit while the acid is absorbed by the plates. It then needs to be fully charged with a de-sulfating charger (high voltage, low amperage).
AGM (absorbed glass mat) maintenance free batteries can take 5 or more hours to absorb acid on initial servicing. And even though they appear charge and ready to go at that point, they only have a surface charge. It takes an additional 5~12 hours on the de-sulfating charger to be at full power.
One of the reasons people have battery problems with their bikes is poor initial service practice. Even when the battery is pre-initialized at the factory, as is the YTZ series Yuasa, they still must be placed on a proper charger before being put into service. The battery also needs periodic maintenance recharge on the dealer's shelf if they have it in stock a long time prior to sale. Once allowed to sef-discharge excessively the battery will never have full life expectancy again.