I have tried to install the latest release version of QGIS (as per today, 3.16) on Mint 20.1 using the same steps from the QGIS page:
sudo apt install gnupg software-properties-common
wget -qO - https://qgis.org/downloads/qgis-2020.gpg.key | sudo gpg --no-default-keyring --keyring gnupg-ring:/etc/apt/trusted.gpg.d/qgis-archive.gpg --import
sudo chmod a+r /etc/apt/trusted.gpg.d/qgis-archive.gpg
sudo add-apt-repository 'deb https://qgis.org/debian focal main'
sudo apt update
sudo apt install qgis qgis-plugin-grass
However, the system installed QGIS 3.10.4 (an old version), which is the same version available on the software manager. I have tried to change the repo link to (https://qgis.org/ubuntu) and still did not install the correct version.
Any reason why this is happening?