For guaranteed pensions, the pension fund has to invest a large portion of the pension fund balance in a low-risk manner, e.g. in bonds. In the current period of low interest rates, only low or even negative returns can be generated on this capital. So high guarantees reduce the returns on the pension fund balance, and retirement assets increase less strongly.
Even in the event of the same retirement assets at the time of retirement, the variable pension model has greater power and generates significantly higher pension payments on average. After all, retirement assets remain invested even after a pension is drawn and can be invested in a more balanced manner.