How is the RSSI value determined exactly?

For a research project I am measuring the RSSI value of packets between two XBee Pro 60mW series 2 wire antenna modules. I have the following situation:

I cover sensor 1’s antenna with my hand. When I send a packet from sensor 1 to sensor 2, the RSSI value is 32. However, when I send a packet from sensor 2 to sensor 1, the RSSI value is much higher: 43.

My question is: why is this not symmetrical? If I put my hand over sensor 1’s antenna, then both sent and received packets should be equally attenuated in my opinion, but clearly that is not the case. How come the sent packet is not attenuated in any way, even though I keep my hand over the antenna? I’m looking for a detailed explanation for this phenomenon as I can’t think of any reason why this would happen. Thank you in advance for your kind help!

The RSSI level the radio reports is the all noise at the antenna port including back ground noise. So no it may not be the same.