Pasting from Office to WordPress is never a clean experience. There are two options: paste as you’re doing and clean up the code later (manually deleting the <strong> </strong> tags and other bugs), or do as Bas Grave suggested and strip all formatting, then use WP to reformat it. The reason is creating more space is that the space character between the tags adds an extra line.
As you can see from the <ol start="2"> tag WordPress is interpreting item #2 as a completely separate list, which is not good for accessibility or search engines. Stripping the formatting and adding it back in carefully is the safest way to ensure things appear as you want them to – and appear cleanly for screen readers and spiders too.