The answer is that they are not marketing it falsely at all. Surprised? I'll explain.
A manufacturer considers 1 Megabyte to be 1000 Kilobytes, 1 Gigabyte to be 1000 Megabytes, 1 Terabyte to be 1000 Gigabytes and so on. This is correct considering that kilo means 1000 and mega means 1000000 (10^6). However, computers calculate on base 2 and to them, 1 MB is actually 1024 kilobytes, 1GB is 1024MB and 1 TB is 1024GB. This difference in the method of computation is responsible for this "missing space."
Lets take an example of a 500 GB hard disk.
From a manufacturer's point of view, the 500GB will have 500*1000*1000*1000 = 500000000000 bytes.
From a computer's point of view, 500GB is actually 500*1024*1024*1024 = 536870912000 bytes.
So, a hard drive that promises to have 500 GB storage space will actually display 465.66GB, 536870912000-500000000000 = 36870912000 bytes (34.34GB) less storage space when connected to a computer.
|Space Promised||Displayed on a computer||Difference|
Take a look at the table given above to see how much space is "lost" due to computers working on a base 2 system. As you can see, with the increase in capacity of the storage device, there is an increase in the missing space.