76 is also common because 80 column displays were the norm for a LONG time. if you are using text-based windows like how many wordpress based editors (like the Borland ones did) you lose 2 characters for the window-frame text characters, and one for the scrollbar. An extra character is then added so you can see the cursor at the end of the line.

The 72 also being sensible if you want to reserve 4 characters for line numbers in a normal terminal session.