Sandisk Ultra 128GB

Can you explain to me why the drive is only: 124,188,164,096 bytes

I assume your getting this number from something that is displaying the formatted usable bytes.  And formatted space doesn’t account for bytes used for the partition table, bytes reserved for recovery, and if you have formated the drive bytes used by the MBR and etc.

Think of it this way, when you weigh yourself the scale shows more than just your body weight.  It includes the weight of the air in your lungs, the food in your stomach, the food in your digestive track, the liquid in your bladder and the liquid in your bladder, plus any clothes you are wearing.  So the numbers on the scale do not show your actual body weight.  Same thing with the bits on a flash drive.