Married filing separately is better?

I filled out Turbotax with Married Filing Jointly... Initially I do mine and my husband's separately to see if I or he owes anything and then I put them into one.

When I did filing jointly, there is an amount around $4k owed to Fed taxes.

I wanted to see what filing separately would do and so I changed the filing status to Married but filing separately and now it says there's only $773 owed and a refund from state.

I'm a little confused, all the information seems to be there, why would we owe less filing separately I thought Joint was better?

Can you simply switch the filing status in the way I did?