Chuckrox

Another way of looking at this is.....

Your asset liability duration mismatch is good for you as long as the yield curve stay upward sloping or gets even steeper. Your liabilities will lose more value than your assets and your surplus will increase.

However, if the yield curve starts to flatten( rates start to fall, in simplistic terms), your liabilities will grow faster than your assets due to higher durations leading to a shrinking surplus.