A tilted glass cutter has the effect of changing the angle of the cutter wheel.
It narrows the angle on one side and increases it on the other side. So on the side tilted away from vertical (which is what happens when you look down the side of the cutter) has an sharper angle with the glass. This is likely to produce chips along the cutting line.
The side which is tilted toward the glass has a more blunt or shallow angle with the glass. This produces high stress along the line.
The combination of these two effects make for a rough edge when broken and for break failures because of the stresses being at angles to the desired vertical fissure line.
Prepared with information from the Fletcher-Terry company