Remember that a capacitive touch screen passes electricity through your finger to detect the touch position. This creates an electrical connection between your finger and the glass of the screen. The more area this connection has, the better the chance of a good connection, and the higher the quality of the signal. A firmer press puts more of your finger on the glass, which gives the screen a larger connection to the glass.
When you touch lightly, the connection is intermittent, similar to the battery cables on your car. If they don't make solid contact with the battery posts, they may or may not provide enough current to start the car.