It's the accumulated value of your pension fund when you retire that determines your pension income. Your cuirrent monthly contributions increase the value of the fund, but you can't directly calculate a pension income from them.
What age are you now? The older you are now, the lower your pension fund will be when you retire. If you accumulate your fund over 20 years, and expect to live for 30 years after retirement, you might expect the pension income to be lower than your contributions!
€400 per month is a projected income, based on a number of assumptions. You need to find out what those are, to understand where the figure is coming from.
Is the €400 adjusted for inflation? €400 in today's money could be worth €600 or €800 or ... per month when you retire.