Here is the contact phone number for the IRS Treasury Offset Program Call Center:  1-800-304-3107.  You can call them to verify the amount of your offset.

If the info from the IRS Treasury Offset Program does not explain the difference, you can also contact the IRS directly to see if they had to make any changes or adjustments to your tax return.  Here is a TurboTax FAQ for contacting the IRS:

https://ttlc.intuit.com/replies/3301592

Here is an FAQ with info about offsets:

https://ttlc.intuit.com/replies/4143297

NOTE:  TurboTax does not receive information from the IRS or the IRS Treasury Offset Program regarding any differences in your tax refund.