Yup, manufacturers provide storage quoted in kilo/mega/giga/tera-bytes which are decimal prefixes. (i.e. 1kB = 1000 bytes)
The computer are binary and uses kibi/mebi/gibi/tebi-bytes prefixes. (i.e. 1KiB = 1024bytes).
To further complicate the matter, JEDEC (the organization that defines memory standards) has the kibi/mebi/gibi prefixes as K, M, and G respectively. Therefore, a 1kB is not the same as 1KiB but 1KB is the same as 1KiB. However, 1MB is not the same as 1MiB but maybe the same as 1MB (according to the JEDEC notation).
Sorry if that was too much information!
As stated above, your 120GB SSD will have 111.76GB usable.