That's quite normal. Interestingly, when quickly reading the text afterwards, you won't detect most of mistakes, right? Only after a longer break, you will find them when reading the text again.

The reason for this is: When formulating a text, you have the text in your short-time memory. Reading only a few words is enough and your brain is immediately adding all the rest words, because you exactly know what you wrote.

Some months ago we had a thread with a link to a magic reading test: You were asked to read a text very fast in order to detect a few prepared mistakes. After reading the text you were asked how many mistakes you did find. The interesting result: Dozens of these little words were missing but weren't detected by you at all. Only the prepared mistakes were found.

I once noticed, that I make such typing mistakes like you, means these missing little words and that I cannot detect most of them, even if I try to read the text carefully in order to correct it. But if I send the same text to me as e-mail and read it again I will find them immediately. This only works, if the text is differently displayed on the screen, means if the words are displayed at other locations, due to a changed line length, for instance.

Kai