the quick answer is that the website is not perfect, and may miss certain events. Like any other tool, it can be wrong.
My history with sites like these is generally positive, but I have been burned into thinking it's an ok day(s), but it turns out to be packed to the gills with people.
Nowadays, I generally just assume everyday is packed, and am pleasantly surprised when it is not. However, I still can use these sites with the grain of salt.