When you use the hardware button or the dedicated multimedia keyboard button to increase or decrease the volume, the resulting volume is always an even number. If the volume was 22 and you increase the volume, it goes up to 24. But if the volume was 21 and you increase the volume, it also goes up to 24. Why can't it set the volume to an odd number?
It's an even number because that gives you a reasonable number of conveniently-accessible volume levels. If the volume increased by only one unit for each press of the volume key, you would have to press the button 100 times to increase the volume from 0 to 100. That's a lot of button pressing.
On the other hand, if the stop points are even numbers, then that means you need to press the button only 50 times to go from 0 to 100. With autorepeat, that takes you from one extreme to the other in around a second and a half, which feels about right.
Okay, but if you want only 50 steps to go from minimum to maximum, why not make the highest volume level 50 instead of 100?
Because 100 feels nicer.
Bonus reading: The Windows 95 volume control almost went to eleven.