... the frequency of false positives and false negatives don't have to be related at all. The fact that WGA might not reject lots of invalid licenses doesn't say anything about how often it rejects valid licenses. Without knowing the details about how WGA works, it's easy for me to imagine that Microsoft engineered the scheme to err on the side of identifying licenses as valid, e.g. to avoid pissing off large corporate customers, as another poster remarked.

For example, I have a method for figuring out in the morning if the Dow Jones Industrial Average will rise or fall by the end of the day. If I look out my window and see investors plummeting out of office buildings, it will go down. Otherwise, it will go up. My success rate for predicting up days is about 50/50, but my success rate for predicting down days is just about perfect!