Sonoff SNZB-03 software

Hello. I have two snzn-03 motion sensors. I bought the first one a year ago and the second one now. they have different software versions. the new one has (v0.5) and the old one (v0.3). they lie next to each other and the new one shows rssi: (-41) and the old rssi: (-82). what can cause such differences? software or components used?. is it possible to upload new software to the old sensor?