Ethel's kind of right but, imho, not really.
First of all, there is the binary counting system and the decimal counting system. Decimal goes from 0 to 9 (10 numbers), and is what we use every day as humans. Computers use binary, 0's and 1's. The manufacturers always quote their numbers in decimal, yet the computer has to use the capacity in binary, so there is a slight difference.
To find the difference divide your number in GB by 1024^3:
1,000,000,000 / 1,073,741,824 = 931MB
This usually accounts for the main reason of the drop in capacity. The difference is due to the way the file system is constructed on the disk, plus the fact that the memory stick's capacity is not precisely 1GB.