Apple measures gigabytes as 1,000,000,000 bytes (in the decimal system) for advertising purposes whereas
dh properly uses binary, converted to decimal, which means it's actually 1,073,741,824 bytes.
If you take 120,000,000,000 (Apple's advertised capacity) and divide it by 1,073,741,824 you get 111.75 GB when properly measured in binary.
If you visit:
and scroll ALL the way to the bottom of the page, you can see footnote #1:
1. 1GB = 1 billion bytes and 1TB = 1 trillion bytes; actual formatted capacity less.