DW1000 received power -100+

Hi
I am working on some projects with dw1000.I found that the minimum received power can only be up to -85 or -87,first path received power can only be up to -92.I see the curve described in the user manual in 4.7,which can receive power up to -100+.
Does the receiving register control the receiving sensitivity?my receive power adjustment is not that low.my configuration 64M,6.8M,preamble len 512 or 256 or 128 have tried.
thanks.