Constrained vertical line draws as 91°...

Dear all,

As per the photo below, when I draw a vertical line and I constrain it vertically by holding down the shift button, its angle is labelled as being 91°.

I know the line I am drawing is vertical, however, this becomes an issue when I am measuring an angle that is not vertical. How do I know the measured angle is accurate, if Illustrator thinks a vertical line is 91°?

Am I missing something here?

Thanks!