There are a number of reasons beside owning the property outright. They can make a bigger down payment than you can, so the total debt is lower. They bought it at a much lower price many years ago. Their total capitalization is much larger than yours, which drives interest rates toward zero. Their tax basis is different than your tax basis would be.

It is straightforward financial math. If they have more capital than you or a different tax basis, they can have a lower monthly cost basis than you.