That is a very common situation and not rare at all.
The stimulus is actually a 2020 stimulus based on your 2020 tax return, but because people needed money right away because of COVID-19, the IRS sent "advance" payments based on the 2019 (or 2018 tax return if 2019 not yet filed when the stimulus payments were sent). After the 2020 tax return is filed in 2021 then the IRS will send a 2nd check if additional payment is due.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**