The Characteristic Curve
The proportional relationship of amperage and voltage is only true for constant temperatures. But you perfectly know that a light bulb gets hot after a while. This is caused by the flowing electric current. As a consequence the U-I-curve also changes! Because of the thermal motion of the particles in the glow wire the charge carriers collide with these particles more often than when it's cold and the particles hardly move. This results in an increase of the electrical resistance of the light bulb. If the the resistance is higher, a much higher voltage is needed to push the charge carriers through the glow wire at the same amperage.
What do you expect; what will the U-I-curve of a light bulb therefore look like?
