Nvidia 16 bit color depth up to GF256 was rendered internally at 16bit precision and looked really bad, while their 32bit was just a marketing bullet point due to 100% performance hit when enabled. 3dfx had 16bit frame buffer, but internally rendered at 32bits and output was dithered resulting in >20bit color
https://www.beyond3d.com/content/articles/59/"Voodoo2 Graphics uses a programmable color lookup table to allow for programmable gamma correction. The 16-bit dithered color data from the frame buffer is used an an index into the gamma-correction color table -- the 24-bit output of the gamma-correction color table is then fed to the monitor or Television."