I think, I may have an explanation, I could be wrong. Suppose at 60, there are 100 beneficiaries in the pool who have purchased annuity. Obviously, the ones having low ( lower) probability of survival will contribute to the mortality credit of the ones that have high(higher) probability of survival. What happens at 65 ?
Regardless of one purchasing the annuity at 60 or 65, definitely the pool would have shrunk (there will be less than 100 in the pool) i.e. the no. of people contributing to the annuity pool would reduce drastically thereby contributing less to the mortality credit. This, of course is in absolute terms.
Hence the incremental mortality credit reduces while the ones surviving would definitely reap higher payout owing to lesser no. of surviving individuals enjoying the increased payout.
The above, to my mind is very much similar to the pass through securities in the mortgage market. Owing to prepayment, you receive a lump sum CF (one time). But you also buy in the prepayment risk, reinvestment risk and contraction risk, not to mention that your interest payout reduces drastically (in absolute terms) even if the market conditions (interest rate) remains unchanged.
Suggestions, Criticism, welcome.