In general, the old age security program is relatively predictable. The projections are really based on demographics, in this case the number of Canadians aged 65 and over, and on the indexation of benefits, which basically ties in with inflation.
If we assume that inflation is around 2%, in the best of times, and that the population aged 65 and over is growing at around 3.5% a year, or slightly less, this growth would remain compatible with the projected expenditures.