There are no tiles with 1 in either grain or material anywhere on the map. There are 2/2 (and some times 2/3 or 3/2) tiles out by themselves in the middle of nowhere surrounded by zero yield tiles. So 2/2 has to be the base, and any aggregate has to be very small and terrain specific because otherwise most river tiles would be 6/6 for example. Every single tile along every river is at least 4/2, so every tile along that river has a base yield of some sort, yet 4/2 with a 2/2 base means they are not getting any material aggregate from neighboring river tiles. This is why I find an aggregate system hard to believe, or at the very least not noticeable enough to be warranted.

Quoting Heavenfall, reply 12If a tile says 4 grain 5 materials, you'll get 4 grain 5 materials in your city. And those are TECHNICALLY gathered from that tile and the 8 around it.

yes, this is how it works...  I would think of it as two different things, each tile has an individual yield that if based on the terrain (or resource) in that tile. In addition the tile has an aggregate yield that is the sum of the individual yields from the 8 tiles surrounding it. This aggregate yield (the sum of the tiles around the one you found on) is what is displayed and dictates your city yields.

You shouldn't need to worry about the actual individual yields of any particular tile as the aggregate number displayed is what will matter from your city, however, know that it is the data of all the tiles around it that is causing it to have the numbers it has. It saves the player from having to do a bunch of math in their heads but still allows for more variation in yields based on that 'perfect' city location. 

Lemme know if you need/want further explanation.

The big thing to keep in mind while thinking about it if you want to understand the minutiae of it is that the yields you are seeing are what you would get if you put a city there. Not that tile's individual yields. Thus you can run into situations where a tile will not have a visible yield but still be contributing to that of a nearby tile.  A great example of this are tiles that you cannot build a city on legally (say it's too close to another city). It will still contribute to other yields but because you cant build a city there you wont see aggregate yields on it.