Why Google changes its algorithm

The article lists a concrete example why Google could change its algorithm:

"Recently, a search for 'French Revolution' returned too many sites about the recent French presidential election campaign — in which candidates opined on various policy revolutions — rather than the ouster of King Louis XVI.

A search-engine tweak gave more weight to pages with phrases like 'French Revolution' rather than pages that simply had both words."

If you want to get high rankings on Google, it's important to know whether you should use your keywords as a phrase or as separate words on your web pages. This can be different for different keywords.