Strange as it seems, it’s possibly because you’re too clever.
According to recent research from the University of Sheffield it is actually very smart to make typos.
Psychologist Tom Stafford, who studies typos at the university, argues that our brains generalize simple component parts, such as turning words into sentences, so it can focus on the more complex tasks of conveying complex ideas.
The research, reported in Wired Magazine, revealed that when you’re reading other people’s work, this ability helps you arrive at meaning faster by using less brain power. However, it seems to work against you when reading your own work.
This is because you already have an idea of what you intended to convey. When you read your own words back it’s difficult to see what’s absent, such as good spelling, as its competing with the version that’s already in your head.
Stafford gives an example of generalization. Imagine driving to a friend’s barbeque. It’s possible that you end up driving to work by accident if your friend’s house includes a section of your daily commute. This is because when you’re generalizing you are operating on instinct and you become blind to details. Likewise, when you’re proofing your own copy, your brain already knows the destination and you’re blind to the typos.
This also accounts for why your readers are more likely to pick up on your errors. They are on the journey for the first time and don’t know the destination, which is why they pay more attention to the details along the way.
Stafford suggests that if you want to catch your own errors such as spelling, then you should try to make your work as unfamiliar as possible. This could mean for example, changing the font or background colour, or printing out and editing by hand.
This makes a lot of sense to me and may account for the effectiveness of many of the proofreading techniques that are out there, such as reading your copy backwards or out loud.