How do you understand the variance part of Formula 8 in the paper?

[wqx854987945]

(sigma_t^2 and sigma_r^2) in formula 8(2) represent the decomposition of the vector. In the second coefficient, the unit matrix represents the vector itself, and the one that is subtracted represents the projection from the conical frustum coordinate system to the world coordinate system in the direction of the axis, so the second term can only represent the variance perpendicular to the direction of the axis.
So, why are the coefficients of the first term and the subtractive coefficients of the second term different in form? Whether the first term should also be divided by the quadratic power of d？