I've been using Chrome for years now (since 2009) and frequently notice that it shows links as visited even when I visited them far far back in the past, even years. For example, on Google I can see all the pages I used to book my Hawaii vacation in February (6 months ago) styled as purple links.
But the weird thing is that Chrome says only 10 weeks of data is kept in history. So then, how then does it know I visited sites way farther back then 10 weeks? I'm curious because it would be a goldmine of data to have years of my own browsing history to look at.
It's not Chrome which is highlighting links as purple in your Google search results: it's Google itself. If you have enabled Web History, Google Web Search itself will keep track of what you search for and click on, and it will highlight those links appropriately in purple using a simple CSS rule.
(Actually, I'm not quite sure, but that's what I believe from my own observations.)