This is caused by the 3 dB rule for return loss and 4 dB rule found in ISO/IEC 11801:2002 and EN50173:2002.
These rules require the tester to ignore the measurement when the attenuation is less than 3 dB or 4 dB dependant up on the parameter being measured.
On a short link, the attenuation will not exceed 3 dB or 4 dB. In this case the entire frequency range falls within the 3 dB and 4 dB rule. The result of this is that the entire measurement is ignored. So how do we report it? We make the limit 0 dB, and then report the worst case value. That is why you occasionally see large headroom figures such as 48.9 dB.
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