I frequently miss typos when I don’t preview even though I proof the post.
Is there a physiological reason for this?
The preview pane lets you see what you’ve typed in a different font, spacing, etc. Sometimes that’s all I need to pick up on an error that I missed originally.
It’s often hard to catch one’s own mistakes. Easy to spot goofs of others.
Human nature?
Your brain is the best pattern matching machine on the surface of the planet. Supercomputers can’t do what it does. Your brain is so good that when it sees words it can fill in the blanks and correct for errors so that the word doesn’t even need to be typed correctly for your brain to figure out what the word is. A good example is the text here which is often circulated around the internet:
http://www.mrc-cbu.cam.ac.uk/personal/matt.davis/Cmabrigde/
It’s even worse when you wrote the text, because your mind will remember what you were trying to write and will recall that instead of reading what you actually typed. By previewing you are at least showing the text in a different format which may trigger your brain to actually process the words in front of you instead of simply recalling them from your memory.
There’s some good info down near the bottom of the page on the above site.
This board notably allows formatting options. For example, in a reply I may bold face something. While I may make an error in formatting, if I preview before posting, the error becomes obvious.
I think you mean psychological.
It would have been funnier if you’d typed “the error becomes obivous.” 
“the error becomes *obivous”
Better?
It isn’t at all uncommon for me to miss a misspelled word in a letter, for example, on the computer screen and have the mistake leap out at me from the printed page.