Every storage medium has much less than the called size: hard discs, USB sticks, SD cards...
It has been so from the beginning of the consumer PC, but it seems so that this disturbs nobody. For my opinion this is a real big deception, but if nobody does something against this marketing lies, it never will be changed.
The fact is: they count wrong!
instead of 1GB = 1,024MB = 1,048,576kB = 1,073,741,824Byte,
they count 1GB is 1,000MB is 1,000,000kB is 1,000,000,000Byte,
so you have a difference of about 70MB for every GB
In addition they subtract a too big amount for formatting and because nobody notices that, then an amount big more or less is drawn off over again.
The result is: you get e.g. on a 2GB called SD card only between 1.6 and 1.7GB usable space!
upshot: you have to buy a bigger SD card or take a smaller map image