Zigbee module question


I’m trying to implement the XBee-PRO® RF Module, for 2.4GHz.
I placed two metal stations on the ground, with 4m distance from each other. My antennas are located 20cm above the surface, and I can’t put them higher from application reasons, and the antenna is attached to a plastic board , on the base top. The plastic board is the inner part of a metal cover for each of the stations.
Each of the antennas were of TaoGlas FXP 72 dipole antanna model.
Then, I run a simple test, transmitting a packet from one station to the other and backward to the first station.

For each packets stream: Standard deviation is rather small though (not more than 1.4). However, the RSSI mean value varies much (up to 12dBm between two consecutive stream series).
I kept on the same setup conditions. What can be the reason? I’ll be thankful for any comment.