Question about the number of slots at airports

I am wondering how the number of slots at an airport are actually determined. I've read multiple things about it on the forum already, that it has to do with things like number of runways and such. However, when I compared some airports I still found some differences I couldn't really explain.

The first thing I noticed was that it isn't as easy as looking at the total number of runways that can be in use at the same time and neither does the total number of runways or even a combination of the two. The two largest airports I could find ORD and DFW are clearly larger than any other airport, but after that the difference between airports with 13, 12, 11, 10 or 9 slots (I haven't looked at any smaller airports) gets blurry.

For example DTW and DAL have both 13 slots, but ATL, LAX and PVG have 12 slots while all these airports have at maximum 4 parallel runways that could be used simultaneously and I don't see a significant difference in other parts of the airport, except that DTW and DAL have 2 additional runways perpendicular to the 4 parallel runways.

Also why does CDG get 11 slots with 4 parallel runways while LAX and PVG get 12 slots with the exactly same runway configuration? The same applies to LHR and EUG having 10 slots with 2 parallel runways while the same configuration at SMF only gives 9 slots.

The same kind of things can be found between SYD and PDX on 9 slots and IND, TUL, OKC, FCO with 10 slots.

And then there are even example where airports with more parallel runways have fewer slots. For example PHX has 3 parallel runways and has 10 slots, but LAS, SFO and JFK have only 2 parallel runways and get 11 slots.

I think there are many more of these examples.

So, I was wondering if someone can explain where these differences come from or should some airports actually get more (or less) slots?

since you searched the board, I am sure you came across this post by Sasha. this should explain most of it - without actually checking any of the examples.

and as always: if you feel an airport should be updated, gather the required information, make a good argument, put it all in an email and send it to as no matter how much support you get here, only the team can change the stats

Thank you so much about the link to that reaction. I've read many topics but after a while not all from top to bottom and must have missed this specific reaction, unfortunately. So, thank you.