Keep in mind that the Internet actually has more bot traffic than human traffic these days. A 50% discrepancy between Google Analytics and access logs could be explained entirely by bot traffic with no ad-blockers involved.
It could be ad-blockers, though. And I want to emphasize a point you mentioned briefly: it very much depends on your target audience. An anecdote from several years ago, around with IE was still the plurality of traffic on most sites but no longer the majority. Two web properties run by the same company, with different audiences. On one of them, IE did not crack the Top 5 in browsers, and was actually outnumbered by Opera. On another, the entire Top 5 browsers were different versions of IE, and Chrome and Firefox combined were less popular than IE6, IE5.5, and AOL's browser (which apparently was a reskinned IE7).
The moral of the story is, what's true for everyone may not be true for you, and vice-versa.