Category Help

Hi. Quick q about Categories. I’m using them on a site to split content into sections, so for example I have:

Blog
> Life
> Projects

Portfolio
> Web
> Print
> Animation

Now the problem I have is how posts are attached to these topics, so if I write a new post for my web design in my portfolio, I would choose Web as the category, but then WordPress would also class this as under the Portfolio right? So if I was to go to /category/portfolio/ it would appear!!! Well if this is the case, what is the difference between choosing just web or web and portfolio as my categories for the post??

Hopefully everyone understands what I mean here. Thanks