Yeah, what is the duty, 10%? I think only the most zealous officer is going to want to spend the time to collect $2 or $3 because someone is a couple bottles heavy.
On my back-to-back cruises last December I wondered about the same issues in the OP. On our turnaround day Customs didn't take our declarations. They barely looked at anything other than our 'in transit' cards and waived us through. Technically if I bought nothing on the first cruise but double my limit on the second, I would be violating the law. But how would Customs know when X was purchased? They wouldn't. Thus, it really was brought home to me that they use discretionary enforcement to the extreme on low-value duty items.