Never has been. Some get a little irate that 50% may not get them an exact number of minutes or indicate that so many mAH of battery power is available.
Again, it's a gross indicator. About as accurate as the gas gauge in your car.
You also ask "Can it be that a battery is only 75% of what a new battery would be." This is the old battery issue. NiMH and LiON batteries lose some capacity with each charge. So an old battery will show a full charge but in reality be some percentage of mAH that a new charged battery is.
There is no maker that does this any different. In case you ask...
What is difference between full charge capacity and design capacity. Can it be that a battery is only 75% of what a new battery would be. And the charge showing 100% is actually a full charge of the 75% capacity that the battery has diminished to.