Thanks for reaching out to the Community, @cloudybeau.
Happy to help out here. QuickBooks Online and QuickBooks Desktop both use the same tax tables for calculating withholding, so if you've noticed a difference in calculations between the two, it may be a configuration issue with the employees. Keep in mind also that the IRS has updated the way that the W-4 calculates withholdings, so that may be contributing to the difference. I recommend reaching out to your accountant to ensure all of your Payroll Setup information is correct. They have the skills and knowledge necessary to make sure you don't have any problems.
For more information on the changes the IRS implemented this year, I recommend checking out: https://quickbooks.intuit.com/learn-support/en-us/set-up-employees/what-s-changing-with-the-federal-...
Hit the Reply button below if you have any other questions. I'll be happy to help. Have a wonderful day!