All memory sticks reserve some space for internal files that they need for normal operation.
Therefore, you always get a little less available space than what is stated.
Here you can see how much space the different sizes actually have:

Stated space	Real space
2GB	1905MB = 1.86GB
4GB	3809MB = 3.72GB
8GB	7619MB = 7.44GB
16GB	14980MB = 14.63GB
32GB	29800MB = 29.10GB
64GB	59500MB = 58.11GB
128GB	120000MB =117.19GB