The fund is increased by contributions and positive investment returns, the latter normally being a major element. On the other hand, the fund is reduced by benefits (including premium payments and other expenses) and negative investment returns.
That is economic reality, with accounting numbers being merely imaginary. In order to understand future funding requirements, we need to concentrate upon cashflows.
Crucially, prudence can only be assessed in relation to the best estimate, the latter typically not known to be presented to UK clients despite required by TAS 300.