This is not terribly important (because X-Plane always does “the right thing”), but…what’s the difference between a trapezoid and a square that’s been tipped over? Well, consider the checkerboard below.
Note that in the “no-perspective” space the height of each row is the same; in the perspective case the bottom rows (which appear farther away) are less tall than the top ones. From a 3-d perspective, the middel image is the equivalent of stretching the square into a trapezoid; the right image is the equivalent of tipping it over a bit.
I mention this because PlaneMaker 920 will allow you to drag the corners of generic instruments to create perspective effects. When you do this, the positioning of the moving aprts follows the “perspective” pattern you see on the right. This means that if you make a trapezoid-shaped needle, the center pivot point of the needle will not be at the centroid of the trapezoid!
(Based on 3-d perspective it should not be, because the wide end of the trapezoid is “closer” – thus the center must be closer to the small side.)