The app breaks the document into separate sentences and shows me one sentence at a time to proofread. That keeps errors from being buried in the middle of a paragraph, or from overlooking them from being either too interested (or too bored, in the case of the n'th time through the material) and overlooking mistakes that way too.
It is also easier to spot deficiencies in sentence construction and correct them, break up sentence which are too long--things like that.
Both stats I quoted above are a bit misleading. I put a feature in my app that shows the five sentences both before and after the current sentence in a separate dialogue. I'm able to check off any of those sentences at the same time I complete the sentence selected to proof. So very short sentences (one to four words), chapter titles, and scene breaks, I go ahead and check off, and they never come up in the text box as a separate sentence to check. Therefore, the percentage completed goes up very quickly as all the short sentences get checked off.
Having the surrounding sentences visible also shows me the context of the current sentence, as sometimes a question about why it is worded a certain way is explained by the context.
About the last 40% generally involves longer, more complicated sentences, which I"m more likely to find a typo buried within, or decide to fiddle with just to improve the structure or perk it up (get rid of a 'was', for example).
So the last 40% of the content generally takes more time to complete than the first 60%, AND I typically fix more things. And when I say the last 40%, of course I'm not talking about the actually last 40% of the book, since I'm pulling random sentences from anywhere. I proofread the last line in the book at about the 20%, and I haven't yet hit the first sentence of the book.
So hopefully I'll finish this process tomorrow, and hopefully my typo ratio stays low. ;-)