When the screen is locked and we move mouse cursor it blurs the background and asks us to enter the password but if we press some random key(say space bar) to get us to the password screen,it considers the space bar as the first character of the password. Intuitively the first key should show us the password screen as in other operating systems like Windows may be Mac also.
But then you'd miss the first keypress when unlocking. Sounds far more annoying