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Maybe I was unclear on that, sorry, my mother tongue is french. (I try hard to make sense... ;) )
When most robots crawl your site, they look for the robots.txt at the root level.
If they dont find it, a 404 error is registered to the error log, when they do, they read it and abide by it.
Whith an error log filled with 404 error from robots looking for this file it is harder to concentrate on "real" 404 errors.