Someone else may be able to answer this better than me, but if the ASOS is not convinced that all the vis drop is solely from precip intensity but also from BLSN or fog, then it will report less then heavy.
I'm not sure how it detects how much of the vis drop is from precip intensity though. I know it can sense falling precip obviously but it somehow distinguishes it from blowing snow too.
In the case of BDR today, not sure why it didn't report heavy snow because winds were light and there's no actual fog.