I am pricing an MBS under three different rate scenarios: a base case, +5bps and -5bps
I compute partial durations on the base case using the wave method (P. Hagan: Calculating Delta Risks and Hedges via Waves) using 5 bps shock
I sum the partial durations and get my wave base duration
If I compute the duration as a result of the prices on the +/- bps scenarios why this duration be different than the sum of my partials on the wave method
The rate scenarios are parallel shifts.