There's a good chance that it's due to the spousal amount credit.
In order to properly calculate your refund or balance due in separate returns, you must enter your spouse's income in the setting up stage of your return. If you leave this entry as zero, TurboTax will assign the spousal amount credit to you.
When you prepare a coupled return, TurboTax uses each spouse's income entries to determine whether the spousal amount credit should be claimed.
https://turbotax.community.intuit.ca/replies/2647826