Hold on. I don't think that bit about Windows vs Manufacturers was right, but I can be wrong.
1 Gigabyte = 1024 Megabytes = 1024*1024 Kilobytes = 1024*1024*1024 Byte. That is how it physically is on the SSD (any storage device), because powers of two and computers go hand in hand.
What marketing people came up with is: "Look, 1024 is confusing, let's make it a 1000". Then you end up with 15 "marketing GB", which is actually 15*1000^3/1024^3 = 13.97 real world GB.
So Windows is right. It's marketing people who market using imaginary capacity.