I’ve deployed 4 Estimote beacons in hall way at a minimum distance of 5m. All the beacons are set with +4dB transmission power and 300ms of advertisement interval. My two iOS devices (iphone-4S) having same App (beacon ranging and displaying RSSI values from beacons in range) at certain point (location) and at same time display two different sets of RSSI values. However, the RSSI values displayed on any iOS device at any point relative to beacon distance seem very normal,i.e. RSSI value from farthermost beacon is low and RSSI value from near beacon is high. Moreover, all the RSSI values displayed in my first iOS device is relatively higher then in my second device at same point of time and location. For eg, If RSSI value from any beacon in my first iOS device is -74dB then in second device it is -84dB. I’ve notice that there is different of -10dB in two devices. I am trying to make a radio map, and if two iOS devices give two different values then my radio map no longer works on different devices.
Welcome to the convoluted world of Bluetooth radio waves, Bluetooth antenna characteristics and so on 
There could be many explanations: the antenna used in both of the iPhones is slightly different (maybe they used a different component), maybe the case is slightly different (which affects the signal), also, holding the device differently between measurements can affect the readings etc.
That’s why indoor positioning using beacons is so hard, and believe me, we know, we’re actually trying to make it work with our Indoor Location 
Well, we’re quite confident we will make it work, but yeah, it takes time and a lot of hard-core data science!