Why angle of parallel lines is not same? opencv c++

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I detected lane lines in opencv and calculated their angles (which are shown by read lines in the image), although they look almost the same angle, the angle calculated by the program shows quiet a difference with left line always greater than right.

I am using arctan(slope) to find angles.

Is is due to the fact that y-axis in MAT matrix is inverted?

I am trying to detect the difference in the lane line angles to detect turns and straight road. How can I do achieve my goal? which I can not right now because lines do not have same(but opposite) angle on the straight road.

Below is the image.