Traffic data is difficult to collect. Tag-based tracking systems like Google Analytics inevitably miss some visitors, and server log analysis isn’t perfectly accurate either. Let’s assume your tracking is Java-based, and perfectly optimised. It will still miss the 10 to 15% of users that routinely browse the wonders of the internet without Java enabled. However, that doesn’t mean you’ve got a total visit count with a 15% error on it.
What you have is a figure that is usually labelled ‘Total Visitors’, but is in fact not that at all. More correctly, it’s a lower bound on total visitors. It’s a measure of total visitor numbers, sure, but it is not the total number of visitors. When the measure goes up, you know visitors have gone up (assuming the fraction of Java-less users remains the same, which is not unreasonable). When the measure goes down, it’s fair to say that traffic has dropped.
The lower bound on total visitors is a very useful thing to know, but it’s also useful to acknowledge that it is not a true and perfect total visitor count. For a start, presenting the real state of affairs to potential investors or advertisers lets you a use bigger best estimate traffic figure than the one presented by your Java-based tracking system. Your website looks more popular. In fact, it probably is more popular than you think if all you are relying on right now is Java-based tracking like that used by Google Analytics.
We believe you should always try and get an idea of how accurate all your figures are. Knowing that protects you from making poor decisions based on poor data and gives you the confidence to move forward from a fully justifiable position, but in cases like the one discussed above, acknowledging inaccuracy in your stats will actually do you a pretty big favour.