I've noticed that a lot of websites use border radiuses differently for different elements. I've noticed some patterns, but I'm not sure of them.
- I noticed a lot of them use completely rounded corners for search bars.
- I noticed whenever they are forms, they never use fully rounded corners.
I noticed that when they have single call-to-action buttons, they're usually rounded.
Twitter search bar and buttons are fully rounded, but the fields are not.
Facebook sign up has sharp corners at the log in but not the registration.
Google search bar has fully rounded corners but buttons aren't.
Is there any reason why some elements might have their corners made round, while others not?