Does my net income dramatically affect my wife's taxes while she is on maternity leave?

My wife was on Mat-Leave for most of 2017.  When I create my tax return and my wife's tax return separately in TurboTax we both have small returns; that is if I do not enter in each other's net income (line 236) in the spouse information section.  Once this has been done we both owe, me a few hundred and my wife a few thousand!  How can this be?  I get the same result if I create our tax returns together.  Can someone enlighten me on what may be going on?