My reading has been that some phone functions use a cellular data connection even if wifi is turned on which is probably why AT&T requires a data package if you have an iPhone. MMS, for example, seems to go over a cell connection at all times and there are some "phone home" functions in the phone which always go over a cellular data connection. Also, keep in mind that the "wifi bars" only indicate that it has a wifi connection. If there's a problem with the connection that affects data throughput, the phone will fall back to the cellular data connection.