I was reading “Ordering Disorder” Grid principles of Web design from Khoi Vinh, in where he mentions that without much explanation.
He got a long copy (8 units wide) inside a column of 10 units, in where originally he was using the most left unit as a left margin and the most left as a right margin. He moved the copy to the beginning of the most left unit, leaving 2 empty units at the far right for the reason of leaving it as far left as possible.
UPDATE: As required, I provide more info to give more context from the book, hope it helps.
Humans have a tendency scan webpages to the far left of the browser window in a F shaped pattern. It can be centered but if you have some data/distracting content on the left of the main content it gets easily prioritized over the content you want to show (at least until you educate your audience)
Some links explaining this:
The individual design is preference of designer. But it can be motivated. Other reasons might include the near impossible task of making justified text in webpages.
TL:DR: People read webpages different from the way they read books.