Cassandra stress-tool column-size does not change

Question

When testing my schema on single-node cluster, I am getting the identical data for each stress-test run I specify my cassandra-stress.yaml file Table and column spec

table_definition: |
  CREATE TABLE files (
    id uuid PRIMARY KEY,
    data blob
  ) 

columnspec:
  - name: data
    size: UNIFORM(10..100)
    population: UNIFORM(1..100B)

So, I specify the column size to be uniform distributed from 10 to 100, but this config results in identical blob data of fixed size in each row

I use this command to run the test
cassandra-stress user profile=..\cqlstress.yaml n=20 ops(insert=5)

What I am doing wrong?
My wish is to have the data of variable length