Two different types of graphics card are available - reference & non reference cards. Reference cards are those, which are manufactured strictly in compliance with the specs laid down by the chip designer (NVIDIA or AMD, whichever the case maybe). Whatever you read upon NVIDIA's website is applicable strictly to this type of video cards only.
In contrast to these cards, non reference cards are modified by the card's manufacturer (read ASUS, Gigabyte, EVGA etc.) in ways like redesigning the layout, changing the cooler, PCB etc. This is mostly done to achieve better performances (through high clock speeds- low voltages) and possibly also to increase the marketability of their products.
This is why we have different models of the same card available at different prices. Thus, you should not be worried over the base clock speed of your card not tallying with those mentioned at NVIDIA's website. It should be doing fine, however, if it is still bugging you, download a benchmarking tool like Unigine Valley Benchmark and run it to obtain the benchmarking report. You can use the results- the fps, temp etc. thus obtained to compare the performance of your card. You may also mail this report to the support staff of your card's manufacturer and hear to what they have to say.