It is expected, the users inside the container are "real" users. They just get offset inside the container and some mapping is applied:
Root inside the container is mapped outside to the user running the container, everything that has the owner "root" inside the container can be read from outside the container as your user.
Everything that is saved as non-root inside the container gets mapped to the first subuid in /etc/subuids for your user + the uid inside the container.
You can change this mapping such that, for example, user 1000 inside the container gets mapped to your user outside the container.
An example:
You have a postgres database inside a container with a volume for the database files. The postgres process inside the container doesn't run as root but instead runs as uid 100 as such it also saves its files with that user.
If you look at the volume outside the container you will get a permission denied error because it is owned by user 100100 (subuids starts at 100000 and usid inside container is 100).
To fix: Either run your inner processes as root, this can often be done using environment variables and has almost no security impact or add --userns keep-id:uid=100,gid=100 to the cmdline to make uid 100 inside the container map to your user instead of root (this creates a new image automatically and takes a while on the first run)