Fiona, congrats!
Take a look at what you've done on your site in terms of textual content, page titles, image tags, meta descriptions, etc. and you'll probably find the answer to your question.
You're smart to focus your
SEO efforts on a limited number of keywords so you don't wind up diffusing the results you already have for these words and phrases.
To learn more about
SEO, browse this section (
Search Engine Optimization (SEO)) - there are a ton of articles, tools, and other tips posted that will help you better understand how to leverage your potential on-site and off-site
SEO.