RF Image rejection test fails at exact frequency. but gets good BER at nearby frequencies.

I was testing my Receiver for various tests. It gave good sensitivity at -116 dBm. Selectivity and blocking tests went well. When got into image rejection test, something weird is happening.

My RF in = 868.500 MHz.

LO_freq = 870 MHz.

Image frequency = 870.500 MHz (I tested it as both modulated signal and continuous wave signal).

The issue is i dont meet BER requirement when image signal is kept at the exact 870.500 MHz. But i meet BER requirement when i test with 870.499 and 870.501 MHz. what could be the possible reason?