We had test Ruckus with few few different device of Android and we found same AP get different result at the exact same location. some device -65, some device -75.
Different phones have different radios with different electronics such as demodulators and antennas and antenna placements. There's also the issue of not calibrating NICs so you might see different levels with the same type of phones.
You can't fix that, but you can take that into consideration when designig a WLAN network.