Then you remember it wrong.
The memory speed is determined by the graphics card itself, and the card doesn't know or care about the speed rating of the chips. They're either fast enough to keep up with the speed the card runs them at, or they aren't. If you want the card to run the memory faster, then you have to find some way to tell it to run the memory faster.
Now, you may not be mistaken when you remember there being a performance increase when the memory was upgraded on that Diamond card... but it had nothing to do with the speed of the chips used. Back in those days, many cards were set up such that the amount of memory was also tied to the memory bus width. For example, the Cirrus 5434 and Tseng Et4000W32 (among many others) supported a 64-bit memory bus when equipped with 2MB... but they usually only came with 1MB, which also halved the memory bus to 32-bit.