this does contradict the governments claim that public sector pensions are inherently unaffordable
Hmmm. I'm sure you are familiar with the phrase "non sequitur".
I note the careful wording "No larger fall in staff numbers than predicted". Are you a politician?
Forecasting annual pension costs in 2060 is fatuous.
Forecasting annual pension costs
between now and 2060 is far from fatuous. Yes, so it is.
I see that the dreaded "range of error" has crept in, and quite right too. Whenever a report is produced, tailored to prove a point or justify some huge overspend, its writers cover themselves with the "range of error", aka "best and worst case" ignoring the fact that this usually renders the report useless, since the true answer lies anyway in between, but only if you're very lucky. (The same methodology, applied to travel demand forecasts, has been used to "justify" pet infrastructure projects since 1950.)