Even with high traffic it can fluctuate. That is why you should not focus on daily earnings. Look at 7-day and 30-day moving averages. You'll have a lot less anxiety!
I took a look at my daily earnings for 2008 thru yesterday (06 Oct) The standard deviation of the daily earnings is equal to almost 1/3 of the average of the daily earnings.
Max equal to about 200% of average value, Min equal to less than 50% of average value. However, since earnings overall are steadily increasing, this isn't quite a fair assessment with regard to the extreme values.
At the the end of the month a lot of advertisers will run out of money and suspend their ads. Over a month my daily rev will fluctuate drasticly.