They are not planned, they just are. If it felt good to add a couple of critters to a nation it was done. PD is not dealt to nations as a way to finetune balance. Some nations have the same PD as they did in Dom2, while some got new setups. If one nation has a powerful PD consider it a feature and a strength of that nation and vice versa. There are PD's that might fall into the category of puny, but that shouldn't be a problem. If your nation has weak PD, and is weak in every other way, ask your friendly neighbour to take your provinces build PD there and recieve all incomes from him
If you are fighting with the AI weak PD should not be a problem, just a predispositioning feature of the particular setup uyou run. Like handicapping tha AI or yourself.
If a particular nation has a strange PD it is of more urgent concern (like drowning uw PD).