Typos...why?

Lately I've been wondering why people make typos as they type.

When I type, I feel as if I type every word correctly, but then I go back and see a lot of typos...but then at other times I don't make any. I spend a lot of time typing for school and other stuff and it has been bugging me to find out if there is any specific reason that people make typos. Could it be a keyboard/program lag or maybe even a mental problem that makes me believe I typed everything right, when in fact I didn't?