In basic terms a guarantee is a promise that aims to protect against an undesirable outcome eg that investment returns are less than x%. It is a guarantee against a level of uncertainty. The variable that underlies the guarantee will change in terms of its magnitude and probability. Hence over time the guarantee is more or less likely to bite hence the degree to which it is onerous changes. It can also change because a companies attitude to risk has changed. If you think about a guaranteed return, if the returns are poor near the time a maturity guarantee applies say, the less time there is for investment returns to improve so the guarantee does not bite.
