when my population is 100% why don't I get 20% tax

same question. It seems population generated by parks does not provide taxes at all. I understand those people living in parks are pennyless homeless but it's still a bug. 

ok, for whoever having the same "problem" I got official answer (getting 11,645/24h from 92614 sims at 20%) :

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20% tax is a generic tax rate for the whole of your citizens' theoretical income. Your city is at full happiness, therefore you will receive 20% of whatever income the City Hall has calculated all of your citizens are making. A 20% tax rate is not simply calculated by the figure of 20% of your population.

If your tax rate is 20%, and you're receiving 11,645, it indicates that the City Hall calculates your citizens are making at least 463,000 Simoleons! Good stuff. Keep it up!

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So the conclusion:

1) "20%" is pure arbitrary number which means nothing and certainly has nothing to do either with "20" of anything or "percentage" at all as defined in elementary math

2) my sims are making really lot of money. It's only I who only gets roughly 2.5% of it. Which explains all their happyness while the city treasure doesn't have enough simeons for new power plant.