With Sitebuilder we track views and unique visitors by IP Address. Google Analytics does it by setting a cookie.
Cookies are known to under count visits, since they might be disabled or otherwise blocked, they may not expire between visits taken to the site over multiple days, or the visitor may leave the page before the cookie is processed.
Meanwhile IP tracking can over-count since automated visits are included and some ISPs assign roving IPs to their customers that change constantly.
This means you're going to get a hit from every bot and spider crawling your site for like search engines including our uptime system status.jonwebdesign.com.au and more.
In general, the exact number of unique human visitors is somewhere towards the middle, between Sitebuilder Insights and Google analytics.
These aren't flaws in either Sitebuilder's or Google's stats, it's just that there will always be differences between server stats and third-party stats.
Learn How to Add Google Analytics to Sitebuilder here.