When there are two vavs it is pronounced as "v" (I think it's always like that, can't think of any exception) but when there is only one vav it doesn't mean it can't be pronounced with that "v" sound.
No idea why it's like that, I never even thought about it.
The "translation" given in this forum WASN'T one of the options. The "correct translation" given in the multiple-choice exercise used הברווזות , NOT הברווזים . The latter one makes more sense, since it uses the plural that most plural words I've seen, use.