The PN71xx NFC controller includes internal AGC function attenuating the Rx signal coming from the antenna to reach optimum receiver input voltage (preventing against signal saturation).
PN71xx Rx path
As stated in the Antenna Design and Matching guide, the Rrx value must be carefully selected to guarantee that the peak voltage at Rxn/Rxp stays below 1.65Vpk (default value of the AGC configuration and recommended to be left unchanged).
The AGC is internally adjusted depending on the field strength.
RF AGC - Rx voltage over field strength
NOTE: to measure voltage on RXP or RXN make sure to use Active probes.