Expressing sizes in informatics can be confusing at times, this is due to the lack of standardization among hardware manufacturers, programs and operating systems regarding the use of prefixes (K, M, G) when expressing binary multiples. A standard does exist, this is what ISO/IEC 80000-13:2008 is about, it defines a standard for the use of prefixes for binary multiples, however, it has not been widely adopted as of the writing of this post.
This confusion originates from the early days of informatics. In computing, the most basic unit of information is the bit, which could be think of as a switch, it has only two states, on and off (1 and 0). Because you can't store but a certain state (on or off) in a bit, a bigger unit was created, the byte, which is just a group of bits. A byte consists of 8 bits, and this allows to represent a value, from 0 to 255.
The SI (International System of Units) defines that K (Kilo) denotes 1000, and M (Mega) represents 1000×1000, this is based in our base-10 numeric system. Originally 1 kilobyte meant 1000 bytes (103) and there was no confusion. However, in the world of computing, because everything is based on the bit, the numeric system in use is base-2. People noticed the convenience of denote 1 kilobyte as 1024 bytes (210), and one megabyte as 1024×1024 (220); instead of the SI Kilo and Mega convention, which denotes 1KB as 103 bytes, and 1MB 106 bytes.