@bamboo has already given a good explanation. I would summarize the core point again: You get the discretionary (low) interest, because you will get that positive interest no matter the actual returns from the funds’ assets. Even in crisis years, you are entitled to this return (except that at some point the fund will need remediation measures, which usually would be temporary higher contributions for the same benefits)
However, you are mentioning your fund is offering different plans. That sounds like the newer “1e plans”, in which case you are fully entitled to the market returns (and risk) with your “Überobligatorisches Sparguthaben” (which is also why your fund likely does not publish a formal discretionary interest rate).