Earlier quoted context omitted.
Google Analytics is hugely overrated. Most people don't use it properly, many browsers block it entirely , and you can usually do a better job just by looking at server logs.
Saying so just tells me that you never been analyzing and optimizing websites with millions of users. Websites on which a whole company depends on. It would be a crazy approach to try and do it via server logs.
Depending on your userbase, the regular traffic data can be off by significant proportions. I've seen pages where the number of logged-in interactions are higher than the number of Google Analytics hits.