jcbermu's answer is good, but I want to approach this from a different angle.
1GB is 1,000,000,000 bytes (powers of 10) and 1,073,741,824 bytes (powers of 2), then: it shows less storage capacity (the powers of 2). Why is it less? If I see for 1GB more storage capacity in powers of 2 than powers of 10.
A storage media -- any storage media -- can store a specific number of accessible bits. Usually in general purpose computing, it's expressed as bytes or some multiple of bytes, but if you start looking at for example memory ICs (integrated circuits, chips), you will see their memory capacity expressed in terms of accessible bits.
A hard disk will store some specific number of bits or bytes which, for technical reasons, are addressed in terms of sectors. For example, a 4 TB drive might have 7,814,037,168 sectors of 512 bytes each, which works out to a storage capacity of 4,000,787,030,016 bytes. That's what you actually get. (In practice, you then lose some of that to the computer's bookkeeping information: file system, journal, partitioning, etc. However, the bytes are still there, you just can't use them to store files, because they are needed to store the data that effectively allows you to store the files.)
Of course, the number 4,000,787,030,016 is somewhat unwieldy. For that reason, we choose to represent this information in some other way. But as jcbermu illustrated, we choose to do so in two different ways: in powers of ten, or powers of two.
In powers of ten, 4,000,787,030,016 bytes is 4.000787030016 * 10^12 bytes, which rounds quite nicely; with four significant digits, it rounds to 4.001 TB, for the SI definition of "tera": 10^12. Our hard disk can store more than 4 * 10^12 bytes, so in SI terms, it is a 4 terabyte storage device.
In powers of two, 4,000,787,030,016 bytes is 3.638694607 * 2^40 bytes, which doesn't round quite so nicely. It also looks like a smaller quantity, because 3.639 is less than 4.001, and that is bad for marketing (who wants to buy a 3.6 TB drive when the manufacturer next door sells a 4.0 TB drive for the same price?). This is the binary prefix 3.6 "tebibytes", where the "bi" indicates that it's a base-two quantity.
In reality, however, it's exactly the same number of bytes; the number is only expressed differently! If you do the math again, you will see that 3.638694607 * 2^40 = 4.000787030016 * 10^12, so you get the same storage capacity in the end.