If you bring an iPhone (anyone's iPhone) into your home briefly and turn on a Location Services app such as Maps, the true location of your router and other nearby wi-fi hotspots may eventually get transmitted to Apple's database of wi-fi hotspots. An iPad without cellular capability needs to access such a database in order to locate itself.
It's my understanding that Apple used to use a company called "SkyHook" to determine the location of wi-fi hotspots, but is no longer doing so. Instead Apple now uses "crowd-sourced" iPhone data to maintain its own database of wi-fi hotspot locations, taking advantage of the iPhone's GPS and cellular circuitry to "geo-tag" nearby wi-fi networks and periodically send their locations to Apple.
Do you remember the recent publicity about whether Apple is "tracking" individual iPhone users? Apple responded with this document:
Apple Q&A on Location Data
I'm not certain whether wi-fi iPads without 3G currently use the same Apple database, but it seems likely that they do.