The S11 measurement Ultra low power

When we measure S11 by VNA, we generally set the sweeping power to -20~0dBm. I have a question. Does this S11 value we get at sweeping power -10dBm still avaiable when the sweeping power is as low as -80dBm?
Actually, I found in my 802.11g receiver design, the optima reveiver sensitivity (input power -78dBm) is not at point where S11 is the lowest.
Can any one explain why?

Just a guess, but it is likely that your receiver front end has a better noise figure when matched to something other than 50 ohms.