I've noticed the same thing in the last couple of weeks. I always uncheck the "remember me" box, because I sign in with multiple numbers (I book travel for my entire family).
But recently, I've noticed that there is an additional step when logging out. Now, you have to click "Do not remember me" on the log out screen. Otherwise, your number will remain in the memory, even if you initially unchecked the box.
Who knows?!