The measurement itself wasn't good.
I looked at the full 2 kHz data which was taken during the time when I was running the arm loss script on the X arm.
The plot below shows the raw data. The X arm was locked and unlocked sequentially several times.
The ASDC power didn't show a significant difference between the state where it is locked and unlocked.
I am not sure why, but It could be because of a misalginment or some kind of mode-mismatching, which can decrease the coupling efficiency of light going into the cavity.
The raw data were analyzed.
I split the ASDC data into two data, (1) low power state, when the cavity is locked (2) high power state, when the cavity is unlocked.
Then each state was averaged to estimate the averaged ASDC power in each case.
The number I obtained are :
ASDC when X arm was locked = 54.77755 cnts
ASDC when X arm was unlocked = 55.45830 cnts
Those numbers correspond to a round trip loss of 78.780778 ppm, which sounds too small for me.
To see what was going on I will look at the trend data.