Welcome to the exiting world of IT.
Here you will encounter the magic that is ones and zeros,
GigaByte (1000^3) represented as GB and GibiByte (1024^3) represented as GiB
Or in other words, the decimal system meeting the dual system, and if your not care full someone might throw in hexadecimal as well.
And then there is that pesky thing called JEDEC defintion (2^30 bytes = 1GB) - same as GiB
In other words, your 128 GB (Jedec) is roughly 125.82 GB (International System of Units)