The problem is that if you have an alarm or something come up and reach for your phone you sometimes hit snooze in your fumbling and may be too busy to take the phone out of your pocket and check it has relocked. I think in this case the the lock code entry screen can be displayed with the phone still in pocket. I'm actually pretty careful to always lock my phone, yet this happens to me about once a week I'd estimate. I often don't notice the phone is off for several hours.