For years, brands and all the content around bicycles referred to bottom bracket height. This is the distance from the ground up to the center of the bottom bracket. This measurement has phased away, and you will notice that bottom bracket drop (BB drop) is what it has been replaced with.
Most of this has to do with the evolution of cycling. There are so many different wheel sizes and combinations of tires. When you measure from the ground up to the frame, it changes drastically depending on the tire and wheel
Bottom bracket drop is the distance from the horizontal axle line down to the center of the bottom bracket. Using this unit brings a level of comparability that is not affected by wheel/tire selection. The larger the number, the closer the bottom bracket is to the ground and the smaller, the higher it is.