The IRS will send a notice to explain it for you. Sometimes they may make adjustments on their own, but they will let you know why. Per IRS Topic 152 "If you receive a refund to which you're not entitled, or for an amount that's more than you expected, don't cash the check until you receive a notice that explains the difference; then follow the instructions on the notice."
You can find details on that here https://www.irs.gov/taxtopics/tc152.html as well.
