Deciding what to invest in is hard, especially in the seed stage. With the market being so hot, investors have to decide quickly which makes it even harder. When investors are agreeing to an 8 million cap on a YC company and a 4 million cap on comparable non-YC company, they are essentially saying that the YC company is twice as likely to succeed, which I don't think is far-fetched. You may also wonder whether this anticipation fulfils itself (a company that seems more likely to succeed may get "better" investors, positive media coverage, early adopters, etc, which may end up helping it becoming successful).
I also think that the increased popularity of convertible notes is a contributing factor. The 8 million cap only becomes 8 million valuation if the company raises the next round at 8 million or above, so in a way it has to live up to its promise in order for the increased cap to take an effect.
And finally, if a company becomes the next Google then the valuation at the seed stage is insignificant. Therefore the valuation just represents the perceived probability of that happening.