It's starting to sound like maybe you haven't used an Android lock. You can repeat dots. You can go backwards. You can switch directions more than once mid-pattern. You would have no way to tell which dot I started on and which dot I ended on.
Furthermore by your own logic: I unlock my phone and leave residue on the screen. After I unlock it, what do I do next? I use my phone, which remember, by YOUR logic, leaves residue on the screen. So please, for everyone here, explain how you see the difference between where I left the lock screen pattern and where I left the frantic back/forth swipes I made during an intense game of fruit ninja. I for one would love to hear the explanation.