Less Threads than Specified in config file

[Philip Moore]

Hey just a quick question around the threading of Multimech,
 I am using it for some performance testing of our application and using the below config, I end up with 2 user groups and 4 threads regardless of number specified,
 the output does scale with the config change e.g If change to 50 for readers do see significant increase in script output.
so was just wondering is there further threading within threads during multimech runs?
or have I missed something?

[global]
run_time = 90
rampup = 0
results_ts_interval = 10
progress_bar = on
console_logging = off
xml_report = on

[user_group-Readers]
threads = 10
script = readers.py

[user_group-Writers]
threads = 2
script = writers.py

$ multimech-run small_test/

   user_groups:  2
   threads: 4

Any help appreciated.
Many Thanks
Phil