More than likely, the act of transmission is affecting the RSSI value. Transmitting over BLE uses the transmitter that is measuring the RSSI ;)
Try testing it with various EMI effects (like shielding, reflection, etc).
Try reducing the polling rate, and see how the timing effects it as well.
I suspect the RX and TX circuits both run through the same, or nearby circuits and thus are causing crossover between the RSSI value and BLE transmissions.
That's a good point, thanks for the information. Looking around online, it seems that RSSI is unreliable during transmission, which is unfortunate as I was hoping to use it to calculate distance within 10 cm of accuracy.
Also, changing the polling rate definitely helps, but the way to really kill the bad data is by not sending it back to the phone.
Yeah, it's too bad RSSI isn't easier to use. You could use it for long-term location calculation (greater than half a second I would think), but that would involve setting the radio to give an accurate RSSI reading. Thus, depending on your polling rate not so useful.