The trailing slash is just following cp’s own special treatment

# before
dir/
    file1
target/
    orig.txt



# no trailing slash - copy *dir* to target
cp -r /path/to/some/dir /my/target

# after
target/
    orig.txt
    dir/            # dir copied to target
        file1



# with trailing slash - copy *contents* of dir to target
cp -r /path/to/some/dir/ /my/target

# after
target/
    orig.txt
    file1            # contents of dir