The S parameters of an attenuator--but what is actually wrong with it?
It is very likely that port 1 of the attenuator has a bad connection, possibly due to a broken connector.  Somewhere, the RF is passing through a capacitive coupling instead of a direct connection.

Evidence:

1) Attenuation is higher than expected - 20dB instead of 10dB

2) The match is asymmetric - port 2 is well matched, port 1 is basically reading an open circuit.

You can verify this with a multimeter.  A healthy attenuator will read some resistance (say, &lt;100 ohm) between the input and output, and similar between each input and ground.  Your attenuator, I believe, will read an open circuit between input and output.