The difference for most conforming loans (sold to US government) is that less than 20% down payment requires property mortgage insurance (PMI), which should add to monthly payments compared to a 20% down mortgage.

Are you seeing PMI costs? Or are they being waived for you? Or is it a non conforming loan that does not require PMI? There’s a bunch of other possible reasons on this website.

https://www.redfin.com/blog/what-is-pmi-insurance/