Not only that, as the numbers' matrix sometimes change the number of lines and columns.
One time they are tight (lots of lines and columns), and one time they are loose (less lines and columns with more space between them).

Of course it is still possible to do some social-engineering of sorts and guess the right number and position (for instance, you are more likely to unlock with the same hand, and thus you probably won't position your hand/thumb *above* the spot you're aiming at, which allows onlookers to remove an specific area for their analysis), but it will take quite a number of unlocks.
Now compare that with regular number- or shape-passwords: they only require one (1) good look to get compromised...