Hi
This is because the investment variance (which you correctly state is calculated on the full asset value) represents only the difference between actual and expected return.
The expected investment return on the assets backing the BEL is needed in order to support the increase in BEL that arises from rolling it forward by one year, so that doesn't fall into profit. Only the excess of any actual investment return achieved over the expected level would become profit (or 'surplus arising').
However, the total investment return on the surplus assets falls into profit, as it just increases (provided positive) the amount of surplus held - and thus contributes to surplus arising. As we have counted only the excess of actual over expected in the 'investment return variance' item, we need then to add in the expected return on the surplus assets in order to be including the total return.
Equivalently, the company could have calculated the actual return on opening surplus (ie using 4.5% rather than 0.5%) for (a) and then performed the 'investment return variance' step only on the assets backing the BEL (ie on the 113.92). We should still get the same total.
Does that make sense?