That happens when you target competitive keywords that others are focusing efforts on, while other 'miscellaneous' keywords aren't appearing or being targeted.

Take almost any string of words from your site and you'll find you often rank #1 for it - minus the ones that people are targeting.

bfas

It's surprising what Google captures in keywords on your site, if you have many posts on a blog for instance, you may be ranking for a thousand different keywords and not realise it.

That's so true Tony, it's odd how some sites will rank for longer tailed keywords not necessarily related to their niche.

Think about it like a pyramid structure. Often if you ranked for the competitive keyword "dog training" you will often rank for "bulldog dog training in mexico" and all the way down the pyramid of lower/no competition keywords pretty easily. - Assuming not many people are directly targeting those phrases.

This is why you often find people ranking for keywords that they clearly haven't optimized for.

I guess the key is tons and tons of unique content...