It means exactly what it says. If you find a signature pattern, the chances are high that this will mark the start of an image file. BUT, its also possible that there is stored data that will match a signature, completely by coincidence. A certain combination of a couple of consecutive pixels of the right colors could produce raw data that is also the same as a signature. It's unlikely, but it is possible. In fact, one of the test data sets does exactly that. It's also possible that garbage data or random data produced from other sources could produce four bytes that match a signature.
Simply put, there's no guarantee that random data can't match any string of data that you might be searching for. It's just the laws of chance.
It's kind of like saying that if you randomly shuffle a deck of cards, it can produce a perfectly ordered pattern. It's highly unlikely, but its still possible.
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