I don't know for sure on Photon but for the Core, positive values indicate an error and +11 would mean that the WiFi chip driver got confused.
I searched a bit for the RSSI range for WICED but did not find anything very useful. There was comment in a file on github saying they fixed a bug in RSSI:
- The wiced_scan_result_t structure now includes a flag to indicate reception of an off-channel scan result
(rather than returning the RSSI of the result as +200dBm)
I don't think that +11 dB would be a meaningful RSSI for normal access points and phones but it could be a relative measurement with a funny 0 dB value. I think the reported readings can only really be interpreted on a relative scale, comparing one reading against the next--they are not calibrated in any way that I know of.