Pension calculator doesn’t quite add up

I recently availed of the Irish Pension Board’s pension calculator, which is widely advertised on the internet, to do some analysis.

I found that the average return on funds for a 23-year-old male over 42 years paying in and 25 years getting paid is 1.7% per annum, that is retiring at 65 and living to 90.

The calculator assumes that the fund is invested at 5% pa during accumulation and 4% pa during drawdown. If this rate was achieved, the pensions would be almost three times that predicted by the pension board calculator. I assume the difference is made up in costs and commission?