There are many reasons why a refund might be lower than expected. The IRS has the info.

https://www.irs.gov/refunds/tax-season-refund-frequently-asked-questions

"Why is my refund different than the amount on the tax return I filed? (updated 2/5/2021)

All or part of your refund, including any amount for the Recovery Rebate Credit, may have been used (offset) to pay off past-due federal tax, state income tax, state unemployment compensation debts, child support, spousal support, or other federal nontax debts, such as student loans. To find out if you may have an offset or if you have questions about an offset, contact the agency to which you owe the debt.

We also may have changed your refund amount because we made changes to your tax return. This may include corrections to any incorrect Recovery Rebate Credit amount. You’ll get a notice explaining the changes. Where’s My Refund? will reflect the reasons for the refund offset when it relates to a change in your tax return.

Tax Topic 203, Refund Offsets for Unpaid Child Support, Certain Federal and State Debts, and Unemplo... has more information about refund offsets."

Here's the reason it would be reduced in reference to SE deferral. That's not a Creditable Refund; it's a deferral for something not yet paid, so it isn't also considered past due.

It's part of parity for the Self-employed to be treated as employers are treated for the Form 7200 provision.

"Employers that have already deposited all or any portion of the employer's share of Social Security tax during the payroll tax deferral period may not subsequently defer payment of the tax already deposited and generate an overpayment of tax"

"Employers that have already paid the employer's share of Social Security tax on wages during the payroll tax deferral period may not subsequently defer the payment of the tax"

Remember that SE = both an employer and an employee for purposes of the Social Security tax.