Hello
this question has been puzzling me a lot as well. After reading the CFAI book, Schweser, this forum and other websites, I can only conclude that the EoC question 23 (#21, p97) of the CFAI book is wrong and I needed to register on this forum to develop my point.
In the example you discussed from reading #20, we see that indeed, when D(A) > D(L) and the rates fall, the surplus increases, which is logical.
However, looking at question 23 and most specifically the fact the CFAI says that this statement is correct:
“When interest rates fall, contingent immunization switches to more active management because the dollar safety margin is higher.”
is incorrect because the question does not mention explicitly D(A). They do mention that D(L) is 12y (The weighted-average duration of Hanover-Green’s liabilities is about 12 years) but nothing wrt to D(A). If we look at the proposed benchmarks they all have a duration below 9Y. So we could either conclude that it is not possible to answer the question, or assuming that one the benchmarks is appropriate, D(A)
The other messages in this topic saying that change in IR does not impact FV of a fixed liability are correct, but have nothing to do with the surplus or the question.
If you look at Schweser p236 of #20, you see that, wrt contingent immunization, “if asset duration and convexity match those of the liability the surplus will be relatively stable”, in other words, there is no “rule” or “principle” indicating that the surplus of a contingent immunized portfolio should move in a particular direction. The only thing true is that under contingent immunization, if the surplus is <=0, you need to immunize, i.e. D(A) = D(L), so you are insensitive to change in IR. If surplus is >0, you switch to active management mode, which allow you to deviate (upward or downward) your D(A) from D(L),
I hope this helps,
Cheers, Romain