Memory speed is a confusing one, most cards today have GDDR5 memory on them which means their effective clock speed is 4 times the default.
So for example my car has 1502 Mhz memory clock, which is 6008 Mhz effective speed. So when I let's say downclock it by 2000 Mhz in OC Guru, it will end up being 1000 Mhz instead of 1502 due to the multiplier. To further confuse the matter, some OC tools are using a 2 times multiplier so for example in EVGA Precision a -502 Mhz downclock (max) results in ~1251 Mhz and some use no multiplier, just the default speed. Which is yet another reason to use GPU-Z when it comes to overclocking.