My refund from the IRS was almost $400 more than expected.

My refund from the IRS was almost $400 more than expected.  It has been 3 weeks and I have not received anything from the IRS to explain.   I filed in March and finally got the refund in October.  I am curious if I made an error or if this is some kind of interest?   My refund was more than $26,000.

Any advice on my next step as I don’t want to make any errors if that is the issue.