Negative difference RX_Power - FP_Power

I am trying to analyze the influence of the geometry of the devices location to the quality of determining timestamps. I use 4.7 “Assessing the quality of reception and the RX timestamp” from dw1000 user manual for that. Function for getting input data for logarithms is
void dwt_readdiagnostics(dwt_rxdiag_t *diagnostics).
When I calculate RX-FP power difference sometimes value is below zero. How does a negative difference sometimes occur and what does it mean about quality of this blink? Is this LOS, NLOS or Multi-Path?
I can’t understand how the sum of the squares of the magnitudes of the accumulator from the estimated highest power portion of the channel, which is related to the receive signal power(CIR_PWR field) can be zero or lower than it’s own part which used in FP_Power calculation.

I don’t know the exact answer to your problem but there appear to be cases where the CIR_PWR = 0. Would this lead to the results you are seeing? I have done some testing and there is a linked thread about the CIR_PWR = 0 issue here.