Computers run on the binary system which is the no. to the power of 2.
for eg : if we want to make a memory card of 4 GB
1 GB= 2^30
2 GB=2*1 GB
8 GB=2*4 GB…..SO ON
but for eg, if u want to make a 7 GB memory card
7 GB=4 GB+2 GB+1 GB
developing 8 GB is much cheaper and less complex then developing 7 GB memory card same is with 3 GB and 11 GB .