I noticed timber and iron RGOs tend to fluctuate wildly between 0-20% unemployment at various stages of the game with no rhyme or reason. When trying to fill my colonies as Sweden for example, at certain times several swedish iron/lumber RGOs will have 20%+ unemployment, then go back down to 0%, then go back up again. Eventually they settle in to 10-15% unemployment later on in the game. All RGOs are NOT at maximum capacity at any point and have plenty of room for more workers at all stages of the game.