Correct, if it has a wifi signal, it shouldn't use your mobile data. In fact, I find it frustrating sometimes with my phone (which does the same) when it's connected to wifi, but very poorly (ex: I'm sitting in my car about to leave my driveway), so it can't get a real connection, and therefore doesn't really receive any data until I turn off the wifi.
Correct, if it has a wifi signal, it shouldn't use your mobile data. In fact, I find it frustrating sometimes with my phone (which does the same) when it's connected to wifi, but very poorly (ex: I'm sitting in my car about to leave my driveway), so it can't get a real connection, and therefore doesn't really receive any data until I turn off the wifi.