Ex-actuary speaking. Shortfalls in insurance and pension funds are the difference between two extremely large numbers:
the value of the fund - this is known pretty accurately, although some items (such as commercial property) may be estimated. Stock prices can, of course, change rapidly, particularly where hedge funds are involved.
the present value of the future liabilities - this is calculated with great precision, but based on estimates of such things as interest rates in 20 years time. What appears to be a very small change in these underlying assumptions (e.g. 0.25% in the interest rate) can make a big difference to the final answer. When you subtract that answer from the fund value (which, in an ideal world, should be at least approximately equal to value of the liabilities), you can get what appear to be (and, in fact, are) huge swings.
Hope this helps
PV
(Hence the joke about the actuary who was asked "what is two times two". Answer - it all depends, what would you like it to be?)