The 18% discount rate would give an approximation to the correct NPVs for projects with all (or most) of the inflows in the first year.
The present value of $1 to be received one year from now, discounted at 18% is: $0.8475
The present value of $1 × (1 – 0.08) (that is, $0.92) to be received one year from now, discounted at 10% is: $0.8364
The former calculation overstates the correct a