One of the properties of circular and elliptical arcs is the "start angle". When I draw a circular arc the meaning of it's start angle is quiet obvious. The start angle of the following arc is 45°:
However for an elliptic arc it is not clear to me what the meaning of a 45° "start angle" would be. The following elliptical arc has a start angle of 45° (set by me in the properties), but what does this start angle mean?
Therefor the angles match with the circular arc in the X/Y axes (0°, 90°, 180°, 270°), because in these corner cases one of the operands (sin(0°/180°), cos(90°/270°)) becomes 0.
A less mathematical explanation is this:
An ellipse is a distorted circle.
Draw a circle in the center of the ellipse with the radius of major axis.
Then construct the intersection of a 45° center ray with the circles circumference and draw a line parallel to the Y axis through the intersection point.
Where the vertical line intersects the ellipse is the calculated start/end point.