From the top comment here: The precision is greatest when GPS is on, but less precise data from cell towers and WiFi sources is used almost any time a Google app is being used on an Android or non-Android phone.

I haven't read the original article since the website doesn't work without off-domain scripts running, but I assume that's why - they do have data for iOS users running Google apps, and that data is often higher resolution than cell tower triangulation. I'd also assume they're getting the cell information as well and correlating the two.