false measurement

I tried to measure signal from my signal generator with AD-FMCOMMS4-EBZ and zynq-7000(zedboard).  But the measurement which is showed in IIO Osciloscope is not true. There are two screenshots of measurement and signal generator in attached file. The faulty thing is that, I am sending "0 dbm"  from signal generator but I read in IIO  as "-9.87dbm".  Balun's insertion loss is 1.4dbm in frequency I want to measure(1001mhz). There is a loss of (9.87-1.4 = 8.37dbm ). I couldn't  understand  what's the reason of this loss. Thank you.