This is a bit simplified, but...
GPUs are produced in big sheets made up of wafers. Due to inconsistencies in the process, some will have various parts (cores or the like) that don't work. So, the more perfect the part, the higher up the food chain it will get placed. Again this is generalization... but a perfect part becomes a xx80, one with some jacked up cores becomes an xx70, even lower becomes xx60, etc. The better the part the higher the price.
What complicates this is that due to price points and volume, they need fewer xx80's and more xx60's (since more people are willing to spend that amount for a mainstream card) so they artificially disable cores to make a card that could have higher the level they need to meet demand.
Some will clock higher than others as well so 3rd party board makers test chips and then bin them to be stock, overclocked, way overclocked, etc within a level of chip like xx80.
So, you get marketing stratification up and down a product line as well as within a specific one.
Then, each year or two they shrink the dies, adding cores, or tweak the archtiecture to make them faster, thus 670, then 770, then 970.
Does that help?