As with all high-level tasks, your brain generalises simple, component parts (like turning letters into words and words into sentences) so it can focus on more complex tasks (like combining sentences into complex ideas).
"We do not catch every detail, we are not like computers or NSA databases," Stafford added.
According to him, when we are proof reading our own work, we know the meaning we want to convey.
Because we expect that meaning to be there, it is easier for us to miss when parts (or all) of it are absent.
"The reason we do not see our own typos is because what we see on the screen is competing with the version that exists in our heads," Stafford told wired.com.
If you want to catch your own errors, you should try to make your work as unfamiliar as possible.
Change the font or background colour or print it out and edit by hand, he suggested.