A car is not a car if it has no wheels or engine, or doors, or exhaust. It’s just a shell, which has no means of travelling. You couldn’t call it a car, because of the things it’s lacking. If you imagine a web page being that shell of a car, and you have been blindfolded, to work out how to categorize it, then you would not put it in the car category. Maybe you would categorize as car parts, salvage, car wreck – but not “car”.
Google works in the same way, when it is trying to understand what your page is about. The on page SEO you perform together with the keyword density of your page help Google to figure out the best fit in the SERPs for your page/site.
Going in blind, the spiders only have your content as a guide, and even though you have a keyword density on a page that means Google ‘should’ be able to work out your niche – it quite often will not categorize you as you thought.
Staying in the theme, let’s say you had a blog about cars. You set up a page that is to be optimized for the keyword “used cars for sale”. If you simply used that keyword all over the article, and didn’t think to put in keywords like “vehicle”, “automobile”, “car owner”, “engine size”, “second hand car prices” and other LSI keywords, then Google would be inclined to think you were merely featuring the subject on your site, and would not consider you as an authority on the subject of used cars for sale.
Ergo, any other sites that have got LSI, where you have not, are likely to rank higher than you.
To get to grips with LSI, the best place to start is Google itself. You can use the Wonder Wheel to throw up great related keywords, and even look at the footer of the search results themselves for related search terms to your search keyword. If Google themselves class these as LSI, then why aren’t you?
If you write your own articles, you should remember to include as many related terms as possible, and avoid using the same keyword over and over again. Make sure the page you are optimizing has enough related content surrounding it, even as far as category names, outbound links and internal links. Google needs to see your page as an authority on that keyword – and will rank you accordingly.