When someone views your site from a mobile device (smartphone, iPad etc), their location in your analytics / statistics is not 100% accurate to the city level, because it is derived from the IP address rather than from using the device's GPS.

Analytics is not a perfect science and as mobile use increases, the accuracy of these reports is actually declining. This phenomena is due to mobile networks use of the ISP's router location. 

It's kind of like the old AOL dialup days when all AOL users showed up as being from Virginia (their router was in Virginia).

See these articles for more detail:

• IP address geolocation reporting is becoming less accurate
• How Bad Are Geolocation Tools? Really, Really Bad

IP = Internet Provider
ISP - Internet Service Provider