Until now I believed that 1024 bytes equals 1 KB (kilobyte) but I was reading on the internet about decimal and binary system.
What you are seeing is a marketing stunt. Since non-technical people don't know the difference between Metric Meg, Gig, etc. against the binary Meg, Gig, etc. marketers for storage will use the Metric calculation, thus 1000 Bytes == 1 KiloByte.
This can cause issues with development or highly technical people so you get the idea of a binary Meg, Gig, etc. which is designated with a bi instead of the standard combination (ex. Mebibyte vs Megabyte, or Gibibyte vs Gigabyte)