Hi Fran,
I'm talking from my observation of typical snooker stances, with chin on cue. It's possible that a higher head position makes it easier to get the 3 points on the same plane as a vertical blind pulled up from the cue.
Here's a typical snooker stance, showing the position of the shoulder right of the cue, from the player's perspective and the humerus at an angle to the cue direction. An isolated bicep contraction here would curve the tip right to left during the stroke, even if the forearm is hanging vertically. Other muscles need to come into play during the stroke to move the cue in a straight line, and this is true also for a non-vertical forearm.
Colin