[Q] on mix-lm?

[Woosung Kim]

Dear Dr. Stolcke,

I am doing some experiments using interpolated LMs, and
I've noticed that mixed LMs give slightly different PPLs
from PPLs that should be. I mean, PPLs calculated by getting
weighted sums after getting respective models' word probs.
Do you have any documentations or explanations how that 'mix-lm' 
works in your toolkit or how it is different from the correct way?
Of course, the best ways would be to look at the source code,
but I am looking for an easier way.
According to my experiments, mix-lm gives better results when
the baseline model (before mixing) is good (PPL less than 300), 
but it gives worse results when it is not good (PPL above 500).

Thanks in advance,
-- 
Woosung Kim