shuffle two parallel text files not giving same lines even if same random source

This is similar to https://unix.stackexchange.com/questions/220390/shuffle-two-parallel-text-files I have: - two large csv files with parallel lines. (they represent 'before' and 'after' states for particular items). The fields are sometimes strings, sometimes numbers. - a sufficiently long random data file to use with shuf when I want to get a matching random sample I thought of:

shuf -n10 --random-source="random.csv" "file1" 
shuf -n10 --random-source="random.csv" "file2"

but these files no longer match. However, if I put line-numbers in front, it solves the problem:

shuf -n10 --random-source="random.csv" <(cat -n "file1") 
shuf -n10 --random-source="random.csv" <(cat -n "file2")

Can someone explain why? here is sample of random.csv

0.293076138
0.446732207
0.552989654
0.16141527
0.099383023
...

Here is a snippet from the two files:

VA,DEFAULT,72.8027,11.9534.....
VA,DEFAULT,61.8356,11.9342....
VA,DEFAULT,61.8356,....

Note that the first two fields are identical in most of the rows in both files. Maybe this is the issue? I don't know shuf well enough.