It really isn't that weird. Marketing departments advertise drives as a certain size even when the real size is smaller. For example,
1,024 Bytes = 1 Kilobyte (KB)
1,024 Kilobytes = 1 Megabyte (MB) (1,024 * 1,024 = 1,048,576 bytes)
1,024 Megabytes = 1 Gigabyte (GB) (1,024 * 1,024 * 1,024 = 1,073,741,824 bytes)
159,987,531,776 bytes = 149GB
161,061,273,600 bytes = 150GB
So, the marketing team looks at the 161 trillion bytes and says "That's 160 gigs", but in reality it is closer to 150GB. The more they choose to play with the math, the farther off the actual numbers will be from the advertised numbers.