If you think all sites are weighed the same way for every query, you might be in for a surprise. The SEM Post has a great story about how the weight of different ranking signals may change for a site based on different factors.
So, for example, the power of various ranking factors may shift depending on the query. The example The SEM Post gives is for Coca-Cola. Their global homepage ranks number 1 for their name, but the company actually only appears in the footer and the title tag/meta description. If content was weighted the same way for all sites, all the time, you might expect a site that uses the Coca-Cola name to rank a little higher (like Coca-Cola's social accounts and articles, for example).
Location is another place where ranking factors can shift - if you're looking for stuff "near me" vs "stuff near Pittsburg," even if you're in Pittsburg you might get different results.
There's also personalization, of course. If you favor certain sites, they're ranked differently than if you didn't favor them. Interesting to think about, eh?