Benefits going out don't equal funding coming in plus portfolio performance.
Part of the problem is how these pensions are calculated. Very often a pension is based on the employee’s highest earning years (using three years is common).
So let’s say you’re a police officer, and over your career you averaged $50,000 per year. Your contributions to the pension system will reflect that number.
But in your three highest earning years you averaged $90,000 per year (you got raises, and worked all the overtime you could). You pension will be based on $90,000, not $50,000.