Over the past few months I've noticed a lot of big name websites that seem to trivialize the "Sign In" design in comparison to the "Sign Up" design, which is much more centrally located, and usually also has something to make it visually stand it (most often being a large, different colored button).
The scenario that seems to often happen is that an user is operating on instinct and habitually clicks on the button, but has to traverse back and then attempt to locate the sign in option somewhere. Though small, this can be an annoying and frustrating to a regular/frequent user. Evernote is one example of what I'm trying to describe. The Sign Up button is pretty big and catches my sight immediately, and I tend to habitually click on it. But when I traverse back, I immediately tend to go towards top right (where Sign In is usually located) only to not find it and then browse the home page for another another second or two before finally finding it.
Is this an user experience trend that is based off the fact that regular users have "learned" where to go to sign in and it's such a small price to pay for more users to sign up that it's worth the trade off? Or is this just an unusual example? Yelp seems to have the same trend going on but it's at least in the top right corner.
tl;dr: Why is it that there isn't equal weight in "Sign In" and "Sign Up" designs?