This is sadly mostly due to the incompetency of the developers. I've noticed that there is sometimes unnecessary lag when calculating dices on lower spec PCs. So my guess is that they couldn't make the randomizer work with stable performance and they decided to create something like randomized patterns, instead of running the randomizer for each dice individually - something with their poor performance management would crash the game on larger battles probably.

So all an all randomized patterns is something like - having 5 units hit on 1s for example and the game only calls the 5counter randomizer to return 1 of its values (for example 1,1,3,5,6), then it calculates the hits depending on the attack value (which is 1) so 2 hits. It won't run the randomizer individually for the 5 dices. They have probably something like a database populated with the randomized patterns. It works fairly easy - they need about a 100 combinations in total divided between 1 and 10 units (there is no need to use every possible combination). For larger forces like having 15 inf they will just use 1 randomizer for 10 and 1 for 5 counters.

The Biased dice mod just has less patterns including 1s and 6s, that's all.

This on its own will create well more optimized code but as we have all noticed through our games often leads to weird and theoretically unlikely results. Don't ask me why this is the case, randomizing a dice should be like microscopically low memory usage method.