Why does it matter whose W-2 it is?

Married Filing Jointly.  Three W-2's: 1 for me, 2 for spouse

 

TurboTax mistakenly identified one of our W-2's as mine.  When I corrected the name on it in TurboTax, the 'tax due' was reduced by $200?  What?!

 

Why does it matter whose it is?  I thought MFJ just added all the incomes together and calculated taxes based on the combined income.  I didn't change anything except the dropdown to select that it belonged to my spouse.