There are generally three types of cards circulated:
Reference Cards - These cards use the same PCB with no modifications though sometimes as with the EVGA SC series, they will change the cooler. These are basically "all the same" and the above advice holds
Special Cards - At the other end of the spectrum we have after market "Souped Up" and tweaked as far as technology will allow. These include cards such as the MSi Lightning, Asus Matrix and EVGA Classified. These represent the top end limit if the technology available. Here the above advice does not hold. There are significant differences between the cards tho not so much that one has a huge advantage over the other.
Non-Reference Cards - This segment represents the bulk of the market by far and includes cards like the Asus Matrix and MSI Gaming Series. Here we see a large variation in component quality which is one reason, for example, that when you read reviews on the GTX 970, just about every one reviewing the Gigabyte WindForce and MSI Gaming shows performance levels above 1500 Mhz and Asus / EVGA do not. The suggestion that these cards are all the same is not based in fact. The suggestion that tho they may have different clock rates in the box, they all OC to the same level is not based in fact.
Looking at this article for example on various 970s, if you focus on the bottom of the page, you will see that each card has different PCI-E sockets, different numbers of power phases, some cool the PCB chips with thermal pads or heat sinks, some do not, some use premium chocks, some do not ... each uses a different method of cooling (Asus uses single channel fan control / MSI uses dual control)
http://www.bit-tech.net/hardware/graphics/2014/09/19/nvidia-geforce-gtx-970-review/1
As a result, they perform differently, they OC differently, they run at different temps, and they respond differently to overclocking. It must be noted however, that the manufacturer's investment in this extra / improved technology diminishes the further down the chain you get from the top card.