Why did they use a 1040-SR form

The 1040SR just has larger print for seniors to make it easier to read.  There is no difference in the calculations.

Since the difference between the refund you expected and the refund you received is exactly $600, perhaps you indicated on your return that you did not get the 2nd stimulus payment and the IRS thinks you did.

You can contact the IRS on the calculation of your refund.  To contact the IRS, please review the steps and information below:

IRS Support Number: (800) 829-1040

To reach a live agent at the IRS support number, follow these steps:

1.      Select your preferred language.

2.      Press 2 for “Personal Income Tax”

3.      Press 1 for “Form, Tax History, or Payment”

4.      Press 3 for “All Other Questions”

5.      Press 2 for “All Other Questions”

6.      Do not enter your SSN or EIN. It will ask you twice; do not enter anything.

7.      Press 2 for “Personal or Individual Tax Questions”

They are available Monday through Friday, from 7:00 a.m. to 7:00 p.m. local time (Alaska & Hawaii follow Pacific Time).

Here are some additional resources you can try:

1. Contact a local IRS office.  Click this link for contact information:  Contact Your Local IRS Office
    
2. The Taxpayer Advocate Service (TAS), which is an independent organization within the IRS. Click this link  Taxpayer Advocate Service (TAS)

The use of the 1040SR is immaterial    it's the same as the 1040 but has bigger type.

Sounds to me like you claimed you didn't get the second $600 stimulus....and they think you did.  Maybe that, or perhaps they are going to issue the extra $600 separately, after they  check up on your data.   The IRS may be sending you a snail-mail in the next two weeks explaining why they changed your refund