I thought on this for a while then realized I think I do understand what's taking place.
When % gold modifiers go into a total of -0% in a particular city they become a flat gold modifier. At that point there are no other % modifiers in play in that city so there's nothing to adjust them to be anything more than +/- the same flat number.
So if your city has an overall total of -%, all % modifiers will only end up being +/- modifiers of the same amount until 0 is exceeded. Ex:
City has base 20 gold income.
It has +5% from one building and -10% modifier from another.
The city would be generating 15 gold. 5+-10% = -5% modifier total = -5 gold total from base amount. 20-5 = 15.
Then if a +1% modifier came in, the 'actual' would not understand that this city is actually in negative total modifiers so +1% would mean +1 gold. (-4% modifier = -4 gold. 20-4=16.)
If you were to build then another building that gave +5%, you'd be going into the positive by one. 1% modifier on 20 base gold = +2 gold. 20+2=22.
Does this sort out what you're experiencing?
No, must be a bug, anything of the +1% buildings you build will not increase the +% from buildings. I count +21% of all buildings i already build in city and where i have the resources on right side (Empire wide), no -% buildings in this city, also the city is in +5% all over. But it only displays +15% from buildings (Lower level in text not the upper 15% should go into the lower as graphic symbol is the same there) as you see... remember i should have +21%, when i build a bridal shop and a market, should give 5 more, i have eggs, dried fish and dried meat for the market and +2% on the bridal shop as resources, or do i need the resources not empire wide and i need it in city directly?
City looks like this: (Hover mouse over the cash)
Base Commerce 66
+ 15% from buildings
+ 10% from civics
+ 24 from Specialists (without%)
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100% of 105 = 105.00
+ 10 from Specialists
+ 4 from Religion
+ 137 from Buildings
- 1 free commerce
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Base Gold 255.00
+ 15% from Buildings (Dont increase dosent matter how many +1% buildings you build)
- 10% for Revolutionary leader
- 30% from Civics
+ 30% from Capital
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Total Gold 267.75
Its a bug i think, the +1% building system is not working and will not give the resource +1% bonuses as told... should be fixed, hurry up i want to game on. ;-)
Ok i added alot of +% buildings in the worldbuilder to the city, none is increasing the gold revenue, so looks really like a bug. Also its not inceasing the +% from buildings in hover text. It still only give the normal building gold ammounts like jewelry shop +4, Grocer +1 and so on... but its not increasing the overall income of the city as counted and displayed +% if you have the required resources, in city or empire wide. (Right side in city screen)
Or should it work the way that the +1% should increase the market normal gold of 1 by 1%? Should be useless then... 1 + 1% = 0.01 = 1.01 Gold when you got eggs? Eggs should give +1% to city income when you build a market and have eggs, i got them in city and empire wide 17 eggs, so i think and hope it should be the way that +1% will increase the city income by 1% not the market income by 1% or? The actual display tells that, the ammount is counted as the income of the city is increased by+%. Otherwise all +1% buildings bonuses shoul be changed to +1 Gold when you have the required resource. So the market will create more income with more resources. Maybe change +% to +1Gold if you cant fix.
Btw, i think i created the largest and most detailed world map for C2C available currently, any way to remove all the units and citys from it fast in Worldbuilder or so and possible to upload it here?