This is because you are working in several different modes. They really are cognitively very different:
Checking the grammar
Checking the spelling
Checking the argument
Checking the overall feel of the thing
It's tough as you have to gear in and out of these modes. It's like having to use a manual transmission.
For some reason I got quite depressed because of the intensities of these modes and the difference between the intensities. A simple walk to the supermarket sorted that out though.