After a couple emails back and forth with Mollom guys (have to say they was really trying to help finding where the issue may be) it seem like there is an issue in the way my plugin handles the client side load balancing and server list updates. As result it hits an older Mollom server instead of the main one. Also i was told that providing the IP information on each call would increase performance as they "build IP-based reputation models on the server side".

So, i am going to work on this changes and then will try to pass through all past spam records and compare the results.
Stay tuned!